Light-Emitting Element, Light-Emitting Device, and Electronic Appliance

ABSTRACT

In a light-emitting element including an EL layer between a pair of electrodes, between an electrode functioning as an anode and a fourth layer having a light-emitting property (light-emitting layer), the EL layer includes at least a first layer having a hole-injecting property (hole-injecting layer), a second layer having a hole-transporting property (first hole-transporting layer), and a third layer having a hole-transporting property (second hole-transporting layer). The absolute value of the highest occupied molecular orbital level (HOMO level) of the second layer is larger than the absolute value of the highest occupied molecular orbital level (HOMO level) of each of the first layer and the third layer. With such a structure, the rate of transport of holes injected from the electrode functioning as an anode is reduced and emission efficiency of the light-emitting element is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current excitation typelight-emitting element in which a light-emitting substance is interposedbetween a pair of electrodes, and a light-emitting device and anelectronic appliance each having such a light-emitting element.

2. Description of the Related Art

In recent years, research and development have been actively conductedon light-emitting elements using electroluminescence. In a basicstructure of such a light-emitting element, a layer containing asubstance having a light-emitting property is interposed between a pairof electrodes. By applying voltage to this element, light emission fromthe substance having a light-emitting property can be obtained.

Such a light-emitting element, which is a self-luminous element, isadvantageous in that pixel visibility is high compared to that of aliquid crystal display, that no backlight is needed, and the like andthought to be suitable for use as a flat panel display element. Inaddition, such a light-emitting element is highly advantageous in thatit can be fabricated to be thin and lightweight. Furthermore, responsespeed being extremely fast is one of the features, as well.

Furthermore, since such a light-emitting element can be formed into afilm form, planar light emission can be easily obtained by forming alarge-area element. Because this feature is difficult to achieve withpoint light sources represented by incandescent light bulbs and LEDs orwith line light sources represented by fluorescent lamps, the utilityvalue for surface light sources, which can be applied to a lightingapparatus and the like, is high.

Such light-emitting elements using electroluminescence are broadlyclassified according to whether the substance having a light-emittingproperty is an organic compound or an inorganic compound. The presentinvention relates to a light-emitting element in which an organiccompound is used for the substance having a light-emitting property. Inthat case, by applying voltage to a light-emitting element, electronsand holes are injected from a pair of electrodes into a layer whichcontains an organic compound having a light-emitting property, so thatcurrent flows therethrough. Then, by recombination of these carriers(electrons and holes), the organic compound having a light-emittingproperty forms an excited state, and emits light when the excited statereturns to a ground state.

Because of such a mechanism, such a light-emitting element is called acurrent-excitation light-emitting element. Note that an excited state ofan organic compound can be a singlet excited state or a triplet excitedstate, and luminescence from a singlet excited state is referred to asfluorescence, and luminescence from a triplet excited state is referredto as phosphorescence.

In improving element characteristics of such a light-emitting element,there are a lot of problems which depend on a substance, and in order tosolve the problems, improvement of an element structure, development ofa substance, and the like have been carried out.

For example, in Non-Patent Document 1: Tetsuo TSUTSUI and eight others,Japanese Journal of Applied Physics, Vol. 38, L1502 to L1504 (1999), ahole-blocking layer is provided so that a light-emitting element using aphosphorescent substance can emit light efficiently. However, asdisclosed in Non-patent Document 1, there are problems in that thehole-blocking layer does not have durability and that the life of thelight-emitting element is extremely short.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the presentinvention to provide a light-emitting element that has high emissionefficiency and a longer life than a conventional light-emitting elementby formation of a light-emitting element having an element structuredifferent from that of the conventional light-emitting element.Furthermore, it is another object of the present invention to provide alight-emitting device and an electronic appliance each having highemission efficiency.

One aspect of the present invention is a light-emitting elementincluding an EL layer between a pair of electrodes. Between an electrodefunctioning as an anode and a fourth layer having a light-emittingproperty (a light-emitting layer), the EL layer includes at least afirst layer having a hole-injecting property (a hole-injecting layer), asecond layer having a hole-transporting property (a firsthole-transporting layer), and a third layer having a hole-transportingproperty (a second hole-transporting layer). The absolute value of thehighest occupied molecular orbital level (HOMO level) of the secondlayer is larger than the absolute values of the highest occupiedmolecular orbital levels (HOMO levels) of the first layer and the thirdlayer.

Note that in the above structure, the absolute value of the HOMO levelof the second layer is larger than the absolute values of the HOMOlevels of the first layer and the third layer by 0.1 eV or more.

Further, another aspect of the present invention is a light-emittingelement including an EL layer between a pair of electrodes. Between anelectrode functioning as an anode and a fourth layer (a light-emittinglayer), the EL layer includes at least a first layer (a hole-injectinglayer), a second layer (a first hole-transporting layer), and a thirdlayer (a second hole-transporting layer). The absolute value of thehighest occupied molecular orbital level (HOMO level) of the secondlayer is smaller than the absolute values of the highest occupiedmolecular orbital levels (HOMO levels) of the first layer and the thirdlayer.

Note that in the above structure, the absolute value of the HOMO levelof the second layer is smaller than the absolute values of the HOMOlevels of the first layer and the third layer by 0.1 eV or more.

That is, by formation of a light-emitting element having one of the twostructures described above, the rate of transport of holes injected fromthe electrode functioning as an anode can be reduced, and therefore theemission efficiency of the light-emitting element can be improved.

Further, besides the above-described structures, between an electrodefunctioning as a cathode and the fourth layer having a light-emittingproperty (a light-emitting layer), the EL layer includes at least afifth layer (a carrier control layer) controlling transport ofelectrons. The fifth layer includes a first organic compound having anelectron-transporting property and a second organic compound having ahole-transporting property. The content of the second organic compoundis less than 50% of the total in mass ratio. Note that more preferably,the concentration is controlled so that the content of the secondorganic compound is greater than or equal to 1 weight % and less than orequal to 20 weight % of the total.

In the above structure, in the case where the fifth layer (carriercontrol layer) functions kinetically, a difference between the absolutevalue of the lowest unoccupied molecular orbital level (LUMO level) ofthe second organic compound and the absolute value of the lowestunoccupied molecular orbital level (LUMO level) of the first organiccompound is 0.3 eV or less. A relationship of P₁/P₂≦3 is satisfied wherethe dipole moment of the first organic compound is P₁ and the dipolemoment of the second organic compound is P₂.

In the above structures, preferably, a metal complex is used for thefirst organic compound and an aromatic amine compound is used for thesecond organic compound.

Further, besides the above-described structure, in the case where thefifth layer (carrier control layer) functions thermodynamically, thefifth layer includes a first organic compound having anelectron-transporting property and a second organic compound having anelectron-trapping property. The content of the second organic compoundis less than 50% of the total in mass ratio. Note that more preferably,the concentration is controlled so that the content of the secondorganic compound is greater than or equal to 0.1 weight % and less thanor equal to 5 weight % of the total. Further, the absolute value of theLUMO level of the second organic compound is larger than the absolutevalue of the LUMO level of the first organic compound by 0.3 eV or more.Furthermore, preferably, a metal complex is used for the first organiccompound and a coumarin derivative or a quinacridone derivative is usedfor the second organic compound.

Further, in the above structures, the thickness of the fifth layer ispreferably greater than or equal to 5 nm and less than or equal to 20nm.

Note that in the above structures, the fourth layer (light-emittinglayer) preferably includes a substance having an electron-transportingproperty.

Further, the present invention also covers a light-emitting device usingthe above-described light-emitting element and an electronic appliancehaving the light-emitting device. The light-emitting device in thisspecification refers to an image display device or a light source(including a lighting apparatus). Further, the category of thelight-emitting device also includes a module in which a connector suchas a flexible printed circuit (FPC), a tape automated bonding (TAB)tape, or a tape carrier package (TCP) is attached to a light-emittingdevice; a module in which a printed wiring board is provided at an endof a TAB tape or a TCP; and also a module in which an integrated circuit(IC) is directly mounted on a light-emitting element by a chip on glass(COG) method.

According to the present invention, the rate of transport of holes thatare carriers of a light-emitting element can be reduced; therefore, theprobability of recombination in the light-emitting layer can beincreased, and the emission efficiency of the light-emitting layer canbe improved. Furthermore, by combining a structure capable of reducingthe rate of transport of electrons with the above structure, alight-emitting element having high efficiency and a long life can beobtained. Further, by applying the light-emitting element of the presentinvention to a light-emitting device and an electronic appliance, alight-emitting device and an electronic appliance each having reducedpower consumption can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B each illustrate a band structure of a light-emittingelement in Embodiment Mode 1.

FIGS. 2A and 2B each illustrate a stacked structure of thelight-emitting element in Embodiment Mode 1.

FIGS. 3A to 3C each illustrate a mode of light emission of thelight-emitting element in Embodiment Mode 1.

FIGS. 4A and 4B each illustrate a band structure of a light-emittingelement in Embodiment Mode 2.

FIGS. 5A and 5B each illustrate a stacked structure of thelight-emitting element in Embodiment Mode 2.

FIGS. 6A and 6B each illustrate a mode of a light emission of thelight-emitting element in Embodiment Mode 2.

FIG. 7 illustrates a band structure of the light-emitting element inEmbodiment Mode 2.

FIG. 8 illustrates a stacked structure of a light-emitting element inEmbodiment Mode 3.

FIGS. 9A and 9B illustrate an active matrix light-emitting device inEmbodiment Mode 4.

FIGS. 10A and 10B illustrate a passive matrix light-emitting device inEmbodiment Mode 4.

FIGS. 11A to 11D each illustrate an electronic appliance in EmbodimentMode 5.

FIG. 12 illustrates a liquid crystal display device using alight-emitting device according to an aspect of the present invention asa backlight.

FIG. 13 illustrates a table lamp using a light-emitting device accordingto an aspect of the present invention.

FIG. 14 illustrates an indoor lighting apparatus using a light-emittingdevice according to an aspect of the present invention.

FIGS. 15A and 15B each illustrate an element structure of alight-emitting element in Example 1.

FIGS. 16A and 16B each illustrate an element structure of alight-emitting element in Example 2.

FIG. 17 illustrates current density vs. luminance characteristics oflight-emitting elements 1 to 4.

FIG. 18 illustrates voltage vs. luminance characteristics of thelight-emitting elements 1 to 4.

FIG. 19 illustrates luminance vs. current efficiency characteristics ofthe light-emitting elements 1 to 4.

FIG. 20 illustrates emission spectra of the light-emitting elements 1 to3.

FIG. 21 illustrates current density vs. luminance characteristics oflight-emitting elements 5 to 8.

FIG. 22 illustrates voltage vs. luminance characteristics of thelight-emitting elements 5 to 8.

FIG. 23 illustrates luminance vs. current efficiency characteristics ofthe light-emitting elements 5 to 8.

FIG. 24 illustrates emission spectra of the light-emitting elements 5 to7.

FIG. 25 illustrates results of continuous lighting tests of thelight-emitting elements 5 to 8 by constant current driving.

FIG. 26 illustrates current density vs. luminance characteristics oflight-emitting elements 9 to 12.

FIG. 27 illustrates voltage vs. luminance characteristics of thelight-emitting elements 9 to 12.

FIG. 28 illustrates luminance vs. current efficiency characteristics ofthe light-emitting elements 9 to 12.

FIG. 29 illustrates emission spectra of the light-emitting elements 9 to11.

FIG. 30 illustrates results of continuous lighting tests of thelight-emitting elements 9 to 12 by constant current driving.

FIG. 31 is a graph illustrating CV characteristics of YGASF.

FIG. 32 is a graph illustrating CV characteristics of YGABP.

FIG. 33 is a graph illustrating CV characteristics of TCTA.

FIG. 34 is a graph illustrating CV characteristics of NPB.

FIG. 35 is a graph illustrating CV characteristics of DNTPD.

FIG. 36 is a graph illustrating CV characteristics of Alq.

FIG. 37 is a graph illustrating CV characteristics of DPQd.

FIG. 38 is a graph illustrating CV characteristics of 2PCAPA.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiment modes of the present invention are describedusing the accompanying drawings. Note that the present invention is notlimited to the description below and a variety of changes can be made informs and details without departing from the spirit and the scope of thepresent invention. Therefore, the present invention is not construed asbeing limited to the description of the embodiment modes given below.

Embodiment Mode 1

In Embodiment Mode 1, a light-emitting element of the present invention,which has a structure that reduces the rate of transport of holes thatare carriers of the light-emitting element, is described.

The light-emitting element in Embodiment Mode 1 includes a firstelectrode functioning as an anode, a second electrode functioning as acathode, and an EL layer provided between the first electrode and thesecond electrode. The EL layer may be provided so that it includes atleast a hole-injecting layer, a first hole-transporting layer, a secondhole-transporting layer, and a light-emitting layer which are stacked inthat order from the first electrode side, and that the highest occupiedmolecular orbital level (HOMO level) of the first hole-transportinglayer is deeper (the absolute value is larger) or shallower (theabsolute value is smaller) than the HOMO levels of the hole-injectinglayer and the second hole-transporting layer. There is no particularlimitation on other layers. Further, in the light-emitting element inEmbodiment Mode 1, when voltage is applied to each electrode so that thepotential of the first electrode 102 is higher than that of the secondelectrode 104, light can be emitted.

Thus, the case is described in which an EL layer 103 includes, from thefirst electrode 102 side, a first layer (hole-injecting layer) 111, asecond layer (first hole-transporting layer) 112, a third layer (secondhole-transporting layer) 113, a fourth layer (light-emitting layer) 114,a fifth layer (electron-transporting layer) 115, and a sixth layer(electron-injecting layer) 116, as shown in FIGS. 1A and 1B.

In FIG. 1A, the EL layer 103 of the light-emitting element is providedso that the highest occupied molecular orbital level (HOMO level) of thesecond layer (first hole-transporting layer) 112 is deeper (the absolutevalue is larger) than the HOMO levels of the first layer (hole-injectinglayer) and the third layer (second hole-transporting layer). With such astructure, the rate of transport of holes during the period frominjection from the first electrode 102 to reach to the fourth layer(light-emitting layer) 114 can be reduced. In this case, specifically,the absolute value of the HOMO level of the second layer 112 ispreferably larger than the HOMO levels of the first layer 111 and thethird layer 113 by 0.1 eV or more.

On the other hand, in FIG. 1B, the EL layer 103 of the light-emittingelement is provided so that the highest occupied molecular orbital level(HOMO level) of the second layer (first hole-transporting layer) 112 isshallower (the absolute value is smaller) than the HOMO levels of thefirst layer (hole-injecting layer) 111 and the third layer (secondhole-transporting layer) 113. Also with such a structure, in a similarmanner to that of the case shown in FIG. 1A, the rate of transport ofholes during the period from injection from the first electrode 102 toreach to the fourth layer (light-emitting layer) 114 can be reduced. Inthis case, specifically, the absolute value of the HOMO level of thesecond layer 112 is preferably smaller than the HOMO levels of the firstlayer 111 and the third layer 113 by 0.1 eV or more.

Note that since the rate of transport of holes injected from the firstelectrode 102 can be reduced in either case of FIG. 1A or 1B, thecarrier balance in the light-emitting element is improved, and higherefficiency of the element can be achieved. Further, which elementstructure of FIG. 1A or 1B is employed is determined depending on theHOMO levels of substances used for the first layer (hole-injectinglayer) 111, the second layer (first hole-transporting layer) 112, andthe third layer (second hole-transporting layer) 113.

Further, particularly in the case where the fourth layer (light-emittinglayer) 114 contains a substance having an electron-transportingproperty, the structure of the present invention, such as the structureof FIG. 1A or 1B, is effective. In the case where the fourth layer(light-emitting layer) 114 contains a substance having anelectron-transporting property, an emission region is formed near theinterface between the fourth layer (light-emitting layer) 114 and thethird layer (second hole-transporting layer) 113. In addition, whencations are generated because of excessive holes in the vicinity of thisinterface, cations serve as a quencher, whereby the emission efficiencydecreases significantly. However, since the rate of transport of holesis controlled in the structure of the present invention, generation ofcations in the vicinity of the fourth layer (light-emitting layer) 114can be suppressed and a decrease in emission efficiency can besuppressed. Accordingly, a light-emitting element with high emissionefficiency can be formed.

A structure of a light-emitting element in Embodiment Mode 1 isdescribed using FIGS. 2A and 2B. A substrate 101 is used as a support ofthe light-emitting element. For the substrate 101, glass, quartz,plastics, or the like can be used, for example.

Note that the above substrate 101 may remain in a light-emitting deviceor an electronic appliance which is a product utilizing thelight-emitting element of the present invention, but may only have afunction of a support of the light-emitting element without remaining inan end product.

For the first electrode 102 formed over the substrate 101, a metal, analloy, an electroconductive compound, a mixture thereof, or the likehaving a high work function (specifically, a work function of 4.0 eV ormore) is preferably used. Specifically, indium oxide-tin oxide (ITO:Indium tin oxide), indium tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide (IZO: indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide (IWZO), and the like are given,for example. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper(Cu), palladium (Pd), titanium (Ti), nitrides of metal materials (e.g.,titanium nitride), and the like are given.

Such materials are generally deposited by a sputtering method. Forexample, indium zinc oxide (IZO) can be deposited by a sputtering methodusing a target in which 1 wt % to 20 wt % zinc oxide is added to indiumoxide; indium oxide containing tungsten oxide and zinc oxide (IWZO) canbe deposited by a sputtering method using a target in which 0.5 wt % to5 wt % tungsten oxide and 0.1 wt % to 1 wt % zinc oxide are added toindium oxide. Alternatively, by application of a sol-gel method or thelike, an inkjet method, a spin coating method, or the like may be usedfor the formation.

Further, in the EL layer 103 formed over the first electrode 102, when acomposite material described later is used as a material for the firstlayer 111 formed in contact with the first electrode 102, any of avariety of metals, alloys, electroconductive compounds, and a mixturethereof can be used as a substance used for the first electrode 102regardless of whether the work function is high or low. For example,aluminum (Al), silver (Ag), an alloy containing aluminum (AlSi), or thelike can also be used.

Alternatively, an element belonging to Group 1 or 2 of the periodictable which is a material having a low work function, that is, an alkalimetal such a lithium (Li) or cesium (Cs) or an alkaline-earth metal suchas magnesium (Mg), calcium (Ca), or strontium (Sr); an alloy thereof(e.g., MgAg or AlLi); a rare earth metal such as europium (Eu) orytterbium (Yb); an alloy thereof; or the like can also be used.

Note that in the case where the first electrode 102 is formed using analkali metal, an alkaline-earth metal, or an alloy thereof, a vacuumevaporation method or a sputtering method can be used. Furtheralternatively, in the case of using a silver paste or the like, acoating method, an inkjet method, or the like can be used.

For the EL layer 103 formed over the first electrode 102, a knownsubstance can be used, and any of low molecular compounds and highmolecular compounds can be used. Note that the substance used to formthe EL layer 103 includes not only a structure formed of only an organiccompound but also a structure including an inorganic compound as a part.

For forming the EL layer 103, a hole-injecting layer that contains asubstance having a high hole-injecting property, a hole-transportinglayer that contains a substance having a high hole-transportingproperty, a light-emitting layer that contains a light-emittingsubstance, an electron-transporting layer that contains a substancehaving a high electron-transporting property, an electron-injectinglayer that contains a substance having a high electron-injectingproperty, and the like are combined with each other as appropriate andstacked.

Note that in Embodiment Mode 1, the EL layer 103 is needed to beprovided so that it includes at least the hole-injecting layer, thefirst hole-transporting layer, the second hole-transporting layer, andthe light-emitting layer which are stacked in that order from the firstelectrode 102 side, and that the highest occupied molecular orbitallevel (HOMO level) of the first hole-transporting layer is deeper (theabsolute value is larger) than the HOMO levels of the hole-injectinglayer and the second hole-transporting layer.

FIGS. 2A and 2B each illustrate the case where, from the first electrode102 side, the first layer (hole-injecting layer) 111, the second layer(first hole-transporting layer) 112, the third layer (secondhole-transporting layer) 113, the fourth layer (light-emitting layer)114, the fifth layer (electron-transporting layer) 115, and a sixthlayer (electron-injecting layer) 116 are stacked in that order in asimilar manner to FIGS. 1A and 1B.

The first layer 111 which is the hole-injecting layer is a layer thatcontains a substance having a high hole-injecting property. As thesubstance having a high hole-injecting property, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be used. Alternatively, as a low molecular organic compound, aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc), copper(II) phthalocyanine (abbreviation: CuPc), or vanadylphthalocyanine (abbreviation: VOPc) can be used.

Alternatively, as examples of a low molecular organic compound, thereare aromatic amine compounds such as4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Further alternatively, a high molecular compound (an oligomer, adendrimer, a polymer, or the like) can be used. For example, there arehigh molecular compounds such as poly(N-vinylcarbazole) (abbreviation:PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′[4-(4-diphenylamino)phenyl]phenyl-N′phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD). Alternatively, a high molecular compound to which acid isadded, such as poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonicacid) (PEDOT/PSS), or polyaniline/poly(styrenesulfonic acid) (PAni/PSS),can be used.

Alternatively, for the first layer 111, the composite material in whicha substance having an acceptor property is contained in a substancehaving a high hole-transporting property can be used. Note that by useof the material in which a substance having an acceptor property iscontained in a substance having a high hole-transporting property, amaterial used to form the electrode can be selected regardless of itswork function. That is, for the first electrode 102, a material with alow work function can also be used instead of a material with a highwork function. Such a composite material can be formed by co-evaporationof a substance having a high hole-transporting property and an acceptorsubstance. Note that in this specification, composition refers to notonly a state where two materials are simply mixed with each other butalso a state where charge can be given and received between materials bymixture of a plurality of materials.

As an organic compound used for the composite material, any of a varietyof compounds such as aromatic amine compounds, carbazole derivatives,aromatic hydrocarbons, and high molecular compounds (oligomers,dendrimers, polymers, and the like) can be used. Note that the organiccompound used for the composite material is preferably an organiccompound having a high hole-transporting property. Specifically, asubstance having a hole mobility of 10⁻⁶ cm²/Vs or more is preferable.However, any substance other than the above substances may be used aslong as it is a substance in which the hole-transporting property ishigher than the electron-transporting property. Hereinafter, organiccompounds which can be used for the composite material are given inspecific terms.

For example, as the organic compounds that can be used for the compositematerial, there are aromatic amine compounds such as MTDATA, TDATA,DPAB, DNTPD, DPA3B, PCzPCA1, PCzPCA2, PCzPCN1,4,4′-bis[N-(1-naphthyl)-N-phenylamino]byphenyl (abbreviation: NPB orα-NPD), andN,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD); carbazole derivatives such as4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl-2,3,5,6-tetraphenylbenzene.

Further, there are aromatic hydrocarbon compounds such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),9,10-bis[2-(1-naphthyl)phenyl)-2-tert-butyl-anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene, and2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene.

Furthermore, there are2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene,pentacene, coronene, 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), and 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA).

Further, as the substance having an acceptor property, an organiccompound such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ) or chloranil, or a transition metal oxide can beused. Alternatively, any of oxides of metals belonging to Groups 4 to 8of the periodic table can be used. Specifically, vanadium oxide, niobiumoxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide,manganese oxide, and rhenium oxide are preferable because of their highelectron-accepting property. Above all, molybdenum oxide is particularlypreferable because it is stable even in the atmosphere, has a lowhygroscopic property, and is easy to handle.

Note that the composite material may be formed using the above highmolecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD and the abovesubstance having an acceptor property and then used for the first layer111.

The second layer 112 which is the first hole-transporting layer and thethird layer 113 which is the second hole-transporting layer containsubstances each having a high hole-transporting property. As thesubstance having a high hole-transporting property, for example, it ispossible to use any of the following low molecular organic compounds:aromatic amine compounds such as NPB (or α-NPD), TPD,4,4′-bis[N-(9,9′-dimethylfluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: DFLDPBi), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]-1,1′-biphenyl(abbreviation: BSPB); 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP);2,7-di(N-carbazolyl)-spiro-9,9′-bifluorene (abbreviation: SFDCz);4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA);N-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF);N,N′-bis[4-(9H-carbazol-9-yl)phenyl-N,N′-diphenylvinyl-4,4′-diamine(abbreviation: YGABP); 1,3,5-tri(N-carbazolyl)benzene (abbreviation:TCzB); 4,4′,4″-tris(N-carbazolyl)triphenylamine (abbreviation: TCTA);and4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD). Alternatively, it is possible to use a highmolecular compound such as PVK, PVTPA, PTPDMA, or Poly-TPD.

Note that the above substances are mainly substances having a holemobility of 10⁻⁶ cm²/Vs or more. However, any known substance other thanthe above substances may be used as long as it is a substance in whichthe hole-transporting property is higher than the electron-transportingproperty.

In Embodiment Mode 1, although the above substances can be used forforming the first layer 111, the second layer 112, and the third layer113, it is necessary to select substances to be used depending on theHOMO levels of the substances so that the highest occupied molecularorbital level (HOMO level) of the substance used for the second layer112 is deeper (the absolute value is larger) or shallower (the absolutevalue is smaller) than the HOMO levels of the substances used for thefirst layer 111 and the third layer 113.

Note that among the above materials, the HOMO level of NPB is −5.27[eV], the HOMO level of YGASF is −5.44 [eV], the HOMO level of YGABP is−5.40 [eV], and the HOMO level of TCTA is −5.38 [eV]. Therefore, in thecase of employing the structure shown in FIG. 1A, a combination inwhich, for example, a composite material of NPB, the HOMO level of whichis −5.27, and molybdenum oxide is used for the first layer 111, YGASF,the HOMO level of which is −5.44, is used for the second layer 112, andNPB, the HOMO level of which is −5.27, is used for the third layer 113is possible.

On the other hand, among the above materials, the HOMO level of NPB is−5.27 [eV] and the HOMO level of DNTPD is −5.06 [eV]. Therefore, in thecase of employing the structure shown in FIG. 1B, a combination inwhich, for example, a composite material of NPB, the HOMO level of whichis −5.27, and molybdenum oxide is used for the first layer 111, DNTPD,the HOMO level of which is −5.06, is used for the second layer 112, andNPB, the HOMO level of which is −5.27, is used for the third layer 113is possible.

Note that with the above-described structure, a band gap is formed bythe first layer 111, the second layer 112, and the third layer 113, andtherefore the rate of transport of holes injected from the firstelectrode 102 can be suppressed. Thus, the amount of holes injected intothe fourth layer 114 can be controlled.

The fourth layer 114 is a light-emitting layer containing a substancehaving a high light-emitting property. For the fourth layer 114, any oflow molecular organic compounds given below can be used.

As a light-emitting material which exhibits bluish light, there areN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA), and the like.

As a light-emitting material which exhibits greenish light emission,there are N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′triphenyl-1,4-phenylenediamineabbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),N-[9,10-bis(1,1′-biphenyl-2-yl)]-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), and the like.

As a light-emitting material which exhibits yellowish light emission,there are rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene(abbreviation: BPT), and the like. Furthermore, as a light-emittingmaterial which exhibits reddish light emission, there areN,N,N′,N′tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,13-diphenyl-N,N,N′,N′tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD), and the like.

Further, the fourth layer 114 may have a structure in which the abovesubstance having a high light-emitting property is dispersed in anothersubstance. Note that in the case of the dispersing, the concentration ofthe substance to be dispersed is set to be preferably 20% or less of thetotal. Further, as a substance in which the substance having alight-emitting property is dispersed, a known substance can be used. Itis preferable to use a substance having a lowest unoccupied molecularorbital level (LUMO level) deeper (the absolute value is larger) thanthat of the substance having a light-emitting property and a highestoccupied molecular orbital level (HOMO level) shallower (the absolutevalue is smaller) than that of the substance having a light-emittingproperty.

Specifically, a metal complex such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]-quinolinato)beryllium(II)(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(IiI)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: Zn(BOX)₂), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂) canbe used.

Alternatively, a heterocyclic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazol(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP) can be used.

Alternatively, a condensed aromatic compound such as9-[4-(N-carbazolyl)]phenyl-10-phenylanthracene (abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), or3,3′,3″-(benzene-1,3,5-triyl)tripyrene (abbreviation: TPB3) can be used.

Alternatively, as a substance in which the substance having alight-emitting property is dispersed, a plurality of kinds of substancescan be used. For example, in order to suppress crystallization, asubstance for suppressing crystallization, such as rubrene, may befurther added. Furthermore, NPB, Alq, or the like may be added in orderto efficiently transfer energy to the substance having a light-emittingproperty. Thus, with a structure in which a substance having a highlight-emitting property is dispersed in another substance,crystallization of the fourth layer 114 can be suppressed. Furthermore,concentration quenching caused by the high concentration of thesubstance having a light-emitting property can also be suppressed.

Further, in particular, among the above substances, a substance havingan electron-transporting property is preferably used so that thesubstance having a light-emitting property is dispersed therein to formthe fourth layer 114. Specifically, it is possible to use any of theabove metal complexes and heterocyclic compounds; CzPA, DNA, and t-BuDNAamong the above condensed aromatic compounds; and further high molecularcompounds to be given later as a substance that can be used for thefifth layer 115.

Alternatively, for the fourth layer 114, a high molecular compound givenbelow can also be used.

As a light-emitting material which exhibits bluish light emission, thereare poly(9,9-dioctylfluorene-2,7-diyl) (abbreviation: POF),poly[(9,9-dioctylfluorene-2,7-diyl-co-(2,5-dimethoxybenzene-1,4-diyl)](abbreviation:PF-DMOP),poly{(9,9-dioctylfluorene-2,7-diyl)-co-[N,N′-di-(p-butylphenyl)-1,4-diaminobenzene]}(abbreviation:TAB-PFH), and the like.

As a light-emitting material which exhibits greenish light emission,there are poly(p-phenylenevinylene) (abbreviation: PPV),poly[(9,9-dihexylfluorene-2,7-diyl)-alt-co-(benzo[2,1,3]thiadiazol-4,7-diyl)](abbreviation:PFBT),poly[(9,9-dioctyl-2,7-divinylenefluorenylene)-alt-co-(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene)], and the like.

As a light-emitting material which exhibits light emission in the rangeof orangish to reddish, there arepoly[2-methoxy-5-(2′-ethylhexoxy)-1,4-phenylenevinylene](abbreviation:MEH-PPV), poly(3-butylthiophene-2,5-diyl), poly{[9,9-dihexyl-2,7-bis(1-cyanovinylene)fluorenylene]-alt-co-[2,5-bis(N,N′-diphenyl(amino) 1,4-phenylene]},poly{[2-methoxy-5-(2-ethylhexyloxy)-1,4-bis(1-cyanovinylenephenylene)]-alt-co-[2,5-bis(N,N′-diphenylamino)-1,4-phenylene]}(abbreviation: CN-PPV-DPD), and thelike.

The fifth layer 115 is an electron-transporting layer that contains asubstance having a high electron-transporting property. For the fifthlayer 115, for example, as a low molecular organic compound, a metalcomplex, such as Alq, Almq₃, BeBq₂, BAlq, Znq, ZnPBO, or ZnBTZ, or thelike can be used. Alternatively, instead of the metal complex, aheterocyclic compound such as PBD, OXD-7, TAZ, TPBI, BPhen, or BCP canbe used. The substances given here are mainly substances having anelectron mobility of 10⁻⁶ cm²/Vs or more. Note that any substance otherthan the above substances may be used for the electron-transportinglayer as long as it is a substance in which the electron-transportingproperty is higher than the hole-transporting property. Further, theelectron-transporting layer is not limited to a single layer but mayhave a stacked structure of two or more layers formed of the abovesubstances.

For the fifth layer 115, a high molecular compound can also be used. Forexample,poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py),poly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-pyridine-6,6′-diyl)](abbreviation:PF-BPy), or the like can be used.

Further, the sixth layer 116 is an electron-injecting layer thatcontains a substance having a high electron-injecting property. For thesixth layer 116, an alkali metal, an alkaline earth metal, or a compoundthereof such as lithium fluoride (LiF), cesium fluoride (CsF), orcalcium fluoride (CaF₂) can be used. Alternatively, a layer formed of asubstance having an electron-transporting property which contains analkali metal, an alkaline earth metal, or a compound thereof,specifically, a layer formed of Alq which contains magnesium (Mg), orthe like may be used. Note that in this case, electrons can be moreefficiently injected from the second electrode 104.

For the second electrode 104, a metal, an alloy, an electroconductivecompound, a mixture thereof, or the like having a low work function(specifically, a work function of 3.8 eV or less) can be used. Asspecific examples of such cathode materials, elements that belong toGroup 1 and 2 of the periodic table, that is, alkali metals such aslithium (Li) and cesium (Cs), alkaline earth metals such as magnesium(Mg), calcium (Ca), and strontium (Sr), alloys containing them (MgAg orAlLi), rare earth metals such as europium (Eu) and ytterbium (Yb),alloys containing them, and the like are given.

Note that in the case where the second electrode 104 is formed using analkali metal, an alkaline-earth metal, or an alloy thereof, a vacuumevaporation method or a sputtering method can be used. Alternatively, inthe case of using a silver paste or the like, a coating method, aninkjet method, or the like can be used

Note that by the provision of the sixth layer 116, the second electrode104 can be formed using any of a variety of conductive materials such asAl, Ag, ITO, and indium tin oxide containing silicon or silicon oxideregardless of whether the work function is high or low. These conductivematerials can be deposited by a sputtering method, an inkjet method, aspin coating method, or the like.

Further, as a formation method of the EL layer 103 in which the firstlayer 111, the second layer 112, the third layer 113, the fourth layer114, the fifth layer 115, and the sixth layer 116 are stacked in thatorder, any of a variety of methods can be employed regardless of whetherthe method is a dry process or a wet process. For example, a vacuumevaporation method, an inkjet method, a spin coating method, or the likecan be used. It is to be noted a different formation method may beemployed for each layer.

The second electrode 104 can also be formed by a wet process such as asol-gel method using a paste of a metal material instead of a dryprocess such as a sputtering method or a vacuum evaporation method or.

In the above-described light-emitting element of the present invention,current flows due to a potential difference generated between the firstelectrode 102 and the second electrode 104 and holes and electronsrecombine in the EL layer 103, so that light is emitted. Then, thisemitted light is extracted through one or both of the first electrode102 and the second electrode 104. Accordingly, one or both of the firstelectrode 102 and the second electrode 104 is/are an electrode having alight-transmitting property.

Note that when only the first electrode 102 is an electrode having alight-transmitting property, light emitted from the EL layer 103 isextracted from the substrate 101 side through the first electrode 102,as shown in FIG. 3A. Alternatively, when only the second electrode 104is an electrode having a light-transmitting property, light emitted fromthe EL layer 103 is extracted from the opposite side to the substrate101 side through the second electrode 104, as shown in FIG. 3B. Furtheralternatively, when the first electrode 102 and the second electrode 104are both electrodes having a light-transmitting property, light emittedfrom the EL layer 103 is extracted to both the substrate 101 side andthe opposite side, through the first electrode 102 and the secondelectrode 104, as shown in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 102 and the second electrode 104 is not limited to the abovestructure. Note that any structure other than the above structure may beused as long as it includes at least the first layer 111 which is thehole-injecting layer, the second layer 112 which is the firsthole-transporting layer, the third layer 113 which is the secondhole-transporting layer, and the fourth layer 114 which is thelight-emitting layer, and substances are selected so that the highestoccupied molecular orbital level (HOMO level) of the substance used forthe second layer 112 is deeper (the absolute value is larger) orshallower (the absolute value is smaller) than the HOMO levels of thesubstances used for the first layer 111 and the third layer 113.

Alternatively, as shown in FIG. 2B, a structure may be employed in whichthe second electrode 104 functioning as a cathode, the EL layer 103, andthe first electrode 102 functioning as an anode are stacked in thatorder over the substrate 101. Note that the EL layer 103 in this casehas a structure in which the sixth layer 116, the fifth layer 115, thefourth layer 114, the third layer 113, the second layer 112, and thefirst layer 111 are stacked in that order over the second electrode 104.

Note that by use of the light-emitting element of the present invention,a passive matrix light-emitting device or an active matrixlight-emitting device in which drive of the light-emitting element iscontrolled by a thin film transistor (TFT) can be manufactured.

Note that there is no particular limitation on the structure of the TFTin the case of manufacturing an active matrix light-emitting device. Forexample, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe formed of both of an n-type TFT and a p-type TFT or only either ann-type TFT or a p-type TFT. Furthermore, there is no particularlimitation on the crystallinity of a semiconductor film used for theTFT. An amorphous semiconductor film may be used, or a crystallinesemiconductor film may be used.

In the light-emitting element described in Embodiment Mode 1, the bandgap is formed by the first layer 111, the second layer 112, and thethird layer 113 which are provided between the first electrode 102 andthe fourth layer 114 which is the light-emitting layer, and thereforethe rate of transport of holes injected from the first electrode 102 canbe suppressed. Thus, the amount of holes injected into the fourth layer114 can be controlled. Accordingly, the carrier balance of the wholelight-emitting element can be improved, and an element having highefficiency can be formed.

Embodiment Mode 2

In Embodiment Mode 2, a light-emitting element of the present invention,which has a structure that reduces the rate of transport of electrons inaddition to the structure described in Embodiment Mode 1, which reducesthe rate of transport of holes, is described.

The light-emitting element in Embodiment Mode 2 includes a firstelectrode, a second electrode, and an EL layer provided between thefirst electrode and the second electrode. The EL layer may be providedso that it includes at least a hole-injecting layer, a firsthole-transporting layer, a second hole-transporting layer, alight-emitting layer, and a carrier control layer which are stacked inthat order from the first electrode side, and that the highest occupiedmolecular orbital level (HOMO level) of the first hole-transportinglayer is deeper (the absolute value is larger) or shallower (theabsolute value is smaller) than the HOMO levels of the hole-injectinglayer and the second hole-transporting layer. There is no particularlimitation on other layers.

Thus, the case is described in which the EL layer 103 includes, from thefirst electrode 102 side, the first layer (hole-injecting layer) 111,the second layer (first hole-transporting layer) 112, the third layer(second hole-transporting layer) 113, the fourth layer (light-emittinglayer) 114, a seventh layer 117 (a carrier control layer), the fifthlayer (electron-transporting layer) 115, and the sixth layer(electron-injecting layer) 116, as shown in FIGS. 4A and 4B.

In FIG. 4A, the EL layer 103 of the light-emitting element is providedso that the highest occupied molecular orbital level (HOMO level) of thesecond layer (first hole-transporting layer) 112 is deeper (the absolutevalue is larger) than the HOMO levels of the first layer (hole-injectinglayer) and the third layer (second hole-transporting layer). With such astructure, the rate of transport of holes during the period frominjection from the first electrode 102 to reach to the fourth layer(light-emitting layer) 114 can be reduced. In this case, specifically,the absolute value of the HOMO level of the second layer 112 ispreferably larger than the HOMO levels of the first layer 111 and thethird layer 113 by 0.1 eV or more.

On the other hand, in FIG. 4B, the EL layer 103 of the light-emittingelement is provided so that the highest occupied molecular orbital level(HOMO level) of the second layer (first hole-transporting layer) 112 isshallower (the absolute value is smaller) than the HOMO levels of thefirst layer (hole-injecting layer) 111 and the third layer (secondhole-transporting layer) 113. Also with such a structure, in a similarmanner to that of the case shown in FIG. 4A, the rate of transport ofholes during the period from injection from the first electrode 102 toreach to the fourth layer (light-emitting layer) 114 can be reduced. Inthis case, specifically, the absolute value of the HOMO level of thesecond layer 112 is smaller than the HOMO levels of the first layer 111and the third layer 113 by 0.1 eV or more.

Note that the rate of transport of holes injected from the firstelectrode 102 can be reduced in either case of FIG. 4A or 4B.

Further, in the case of either structure of FIG. 4A or 4B, by theprovision of the seventh layer 117 (carrier control layer) between thefirst electrode 102 and the fourth layer (light-emitting layer) 114, therate of transport of electrons during the period from injection from thesecond electrode 104 to reach to the fourth layer (light-emitting layer)114 can be reduced.

This improves the balance of carriers (electrons and holes) thatrecombine in the fourth layer (light-emitting layer) 114, and higherefficiency of the element can be achieved. Note that which elementstructure of or FIG. 4A and 4B is formed is determined depending on theHOMO levels of substances used for the first layer (hole-injectinglayer) 111, the second layer (first hole-transporting layer) 112, andthe third layer (second hole-transporting layer) 113.

Further, particularly in the case where the fourth layer (light-emittinglayer) 114 contains a substance having an electron-transportingproperty, the structure of the present invention, such as the structureof FIG. 4A or 4B, is effective. In the case where the fourth layer(light-emitting layer) 114 contains a substance having anelectron-transporting property, an emission region is formed near theinterface between the fourth layer (light-emitting layer) 114 and thethird layer (second hole-transporting layer) 113. In addition, whencations are generated because of excessive holes in the vicinity of thisinterface, cations serve as a quencher, whereby the emission efficiencydecreases significantly. However, since the rate of transport of holesis reduced in the structure of the present invention, generation ofcations in the vicinity of the fourth layer (light-emitting layer) 114can be suppressed and a decrease in emission efficiency can besuppressed. Accordingly, a light-emitting element with high emissionefficiency can be formed.

In Embodiment Mode 2, the first electrode 102 functions as an anode andthe second electrode 104 functions as a cathode. In other words, whenvoltage is applied to each electrode so that the potential of the firstelectrode 102 is higher than that of the second electrode 104, light canbe emitted.

Next, a structure of a light-emitting element in Embodiment Mode 2 isdescribed using FIGS. 5A and 5B. The substrate 101 is used as a supportof the light-emitting element. Note that for the substrate 101, glass,quartz, plastics, or the like can be used, for example.

For the first electrode 102 formed over the substrate 101, a metal, analloy, an electroconductive compound, a mixture thereof, or the likehaving a high work function (specifically, a work function of 4.0 eV ormore) is preferably used, and a substance similar to those described inEmbodiment Mode 1 can be used.

Further, in the EL layer 103 formed over the first electrode 102, thestructure of the first layer (hole-injecting layer) 111, the secondlayer (hole-transporting layer) 112, the third layer (hole-transportinglayer) 113, and the fourth layer (light-emitting layer) 114 which arestacked in that order from the first electrode 102 side, and a formationmethod and a material which can be used for each layer are similar tothose of Embodiment Mode 1. Therefore, description thereof is omitted inEmbodiment Mode 2.

In addition to the structure described in Embodiment Mode 1, EmbodimentMode 2 has a feature of providing, between the fourth layer(light-emitting layer) 114 and the second electrode 104, the seventhlayer (hereinafter, referred to as a carrier control layer) 117 thatreduces the rate of transport of carriers (electrons); however, for thestructure of the carrier control layer, two kinds of methods (a methodfor kinetically controlling the rate of transport of carriers and amethod for thermodynamically controlling the rate of transport ofcarriers) can be used.

As the first method, the case of kinetically reducing the rate oftransport of carriers (electrons) by the seventh layer (carrier controllayer) 117 is described. FIGS. 6A and 6B are conceptual diagramsthereof.

The EL layer 103 is formed between the second electrode 104 and thesecond electrode 104. As a plurality of layers included in the EL layer103, from the first electrode 102 side, the first layer (hole-injectinglayer) 111, the second layer (hole-transporting layer) 112, the thirdlayer (hole-transporting layer) 113, the fourth layer (light-emittinglayer) 114, the seventh layer (carrier control layer) 117, the fifthlayer (electron-transporting layer) 115, and the sixth layer(electron-injecting layer) 116 are formed in that order.

The seventh layer (carrier control layer) 117 is formed of two or morekinds of organic compounds. Here, the case where the seventh layer(carrier control layer) 117 is formed of two kinds of organic compounds,a first organic compound 201 and a second organic compound 202, as shownin FIG. 6B, is described. Note that an organic compound having a highelectron-transporting property (an electron-transporting organiccompound) is used as the first organic compound 201 and an organiccompound having a high hole-transporting property (a hole-transportingorganic compound) is used as the second organic compound 202.

Further, the organic compounds used for the second organic compound 202and the first organic compound 201 have LUMO levels close to each other.Specifically, a difference between the absolute value of the lowestunoccupied molecular orbital level (LUMO level) of the second organiccompound 202 and the absolute value of the LUMO level of the firstorganic compound 201 is preferably 0.3 eV or less, more preferably 0.2eV or less. That is, preferably, electrons that are carriers are easilytransported between the first organic compound 201 and the secondorganic compound 202.

In this case, since the second organic compound 202 has a LUMO level asclose as that of the first organic compound 201, electrons can beinjected. The rate (v₁) of electron injection from the first organiccompound 201 having an electron-transporting property into the secondorganic compound 202 having a hole-transporting property or the rate(v₂) of electron injection from the second organic compound 202 into thefirst organic compound 201 is smaller than the rate (v) of electroninjection between the first organic compounds 201.

Thus, by forming the seventh layer 117 using the first organic compound201 having an electron-transporting property and the second organiccompound 202 having a hole-transporting property, the rate of transportof electrons in the seventh layer 117 can be reduced compared to thecase where the seventh layer 117 is formed of only the first organiccompound 201. That is, by forming the seventh layer 117 using the firstorganic compound 201 and the second organic compound 202, the rate oftransport of carriers (electrons) in the seventh layer 117 can bereduced.

Note that in the case where the seventh layer 117 is formed of the firstorganic compound 201 and the second organic compound 202, theconcentration is preferably controlled so that the content of the secondorganic compound 202 is less than 50% of the total in mass ratio.Further preferably, the concentration is controlled so that the contentof the second organic compound 202 is greater than or equal to 1 weight% and less than or equal to 20 weight % of the total.

Note that as the first organic compound 201 contained in the seventhlayer 117, specifically, there are metal complexes such as Alq, Almq₃,BeBq₂, BAlq, Znq, ZnPBO, and ZnBTZ; heterocyclic compounds such as PBD,OXD-7, TAZ, TPBI, BPhen, and BCP; and condensed aromatic compounds suchas CzPA, DPCzPA, DPPA, DNA, t-BuDNA, BANT, DPNS, DPNS2, and TPB3.Alternatively, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py) orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-pyridine-6,6′-diyl)](abbreviation:PF-BPy) can be used.

Further, for the second organic compound 202 contained in the seventhlayer 117, specifically, a condensed aromatic hydrocarbon such as9,10-diphenylanthracene (abbreviation: DPAnth) or6,12-dimethoxy-5,11-diphenylchrysene; or an aromatic amine compound suchas N,N-dipheyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), NPB (or α-NPD), TPD, DFLDPBi, or BSPB can beused. Alternatively, a high molecular compound such as PVK, PVTPA,PTPDMA, or Poly-TPD can be used.

The above materials are combined with each other to form the seventhlayer 117, and accordingly transport of electrons from the first organiccompound 201 to the second organic compound 202 or from the secondorganic compound 202 to the first organic compound 201 is suppressed sothat the rate of transport of electrons of the seventh layer 117 can besuppressed. Further, since the seventh layer 117 has a structure inwhich the second organic compound 202 is dispersed in the first organiccompound 201, crystallization or aggregation over time is not easilycaused. Thus, the above-described effect of suppressing electrontransport does not easily change over time. As a result, also, thecarrier balance does not easily change over time. This leads to anincrease in the life of the light-emitting element, that is, improvementin reliability.

Note that among the combinations described above, a combination of ametal complex as the first organic compound 201 and an aromatic aminecompound as the second organic compound 202 is preferable. A metalcomplex has a large dipole moment as well as a highelectron-transporting property, whereas an aromatic amine compound has arelatively small dipole moment as well as a high hole-transportingproperty. Thus, by combination of substances whose dipole momentsgreatly differ from each other, the above-described effect ofsuppressing electron transport can be further increased. Specifically,where the dipole moment of the first organic compound 201 is P₁ and thedipole moment of the second organic compound 202 is P₂, a combinationsatisfying P₁/P₂>3 is preferable.

For example, the dipole moment of Alq that is a metal complex is 9.40debye, and the dipole moment of 2PCAPA that is an aromatic aminecompound is 1.15 debye. Accordingly, in the case where an organiccompound having an electron-transporting property, such as a metalcomplex, is used as the first organic compound 201 and an organiccompound having a hole-transporting property, such as an aromatic aminecompound, is used as the second organic compound 202, P₁/P₂>3 ispreferably satisfied.

Further, an emission color of the second organic compound contained inthe seventh layer 117 and an emission color of the substance having ahigh light-emitting property which is contained in the fourth layer(light-emitting layer) 114 are preferably similar colors. Specifically,a difference between the peak value of the emission spectrum of thesecond organic compound and the peak value of the emission spectrum ofthe substance having a high light-emitting property is preferably within30 nm. The difference within 30 nm allows the emission color of thesecond organic compound and the emission color of the substance having ahigh light-emitting property to be similar colors. Therefore, if thesecond organic compound emits light due to a change in voltage or thelike, a change in emission color can be suppressed.

As the second method, the case of thermodynamically reducing the rate oftransport of carriers (electrons) by the seventh layer (carrier controllayer) 117 is described. FIG. 7 is a conceptual diagram thereof (a banddiagram).

The EL layer 103 is included between the first electrode 102 and thesecond electrode 104. As a plurality of layers included in the EL layer103, from the first electrode 102 side, the first layer (hole-injectinglayer) 111, the second layer (hole-transporting layer) 112, the thirdlayer (hole-transporting layer) 113, the fourth layer (light-emittinglayer) 114, the seventh layer (carrier control layer) 117, the fifthlayer (electron-transporting layer) 115, and the sixth layer(electron-injecting layer) 116 are formed in that order.

The seventh layer (carrier control layer) 117 is formed of two or morekinds of organic compounds. Here, the case where the seventh layer(carrier control layer) 117 is formed of two kinds of organic compounds,a first organic compound and a second organic compound is described.Note that an organic compound having a high electron-transportingproperty (an electron-transporting organic compound) is used as thefirst organic compound and an organic compound having a function oftrapping electrons (an electron-trapping organic compound) is used asthe second organic compound.

Further, the organic compounds used as the first organic compound andthe second organic compound have LUMO levels that are greatly differentfrom each othel Specifically, the absolute value of the lowestunoccupied molecular orbital level (LUMO level) of the second organiccompound is preferably larger than the absolute value of the LUMO levelof the first organic compound by 0.3 eV or more.

As shown in FIG. 7, a hole is injected from the first electrode 102 intothe fourth layer (light-emitting layer) 114 through the first layer 111,the second layer 112, and the third layer 113. On the other hand, anelectron is injected from the second electrode 104 into the seventhlayer (carrier control layer) 117 through the sixth layer 116 and thefifth layer 115. Since the seventh layer 117 is formed of the firstorganic compound having an electron-transporting property and the secondorganic compound having an electron-trapping property, an electroninjected into the seventh layer 117 enters the LUMO level of the secondorganic compound, not that of the first organic compound. Accordingly,the rate of transport of electrons can be reduced.

Thus, by forming the seventh layer 117 using the first organic compoundhaving an electron-transporting property and the second organic compoundhaving an electron-trapping property, the rate of transport of electronsin the seventh layer 117 can be reduced compared to the case where theseventh layer 117 is formed of only the first organic compound. That is,by forming the seventh layer 117 using the first organic compound andthe second organic compound, rate of transport of carriers (electrons)in the seventh layer 117 can be reduced.

Note that in the case where the seventh layer 117 is formed of the firstorganic compound and the second organic compound, the concentration iscontrolled so that the content of the second organic compound ispreferably less than 50% of the total in mass ratio. Further preferably,the concentration is controlled so that the content of the secondorganic compound is greater than or equal to 0.1 weight % and less thanor equal to 5 weight % of the total.

Note that for the first organic compound contained in the seventh layer117, specifically, a metal complex such as Alq, Almq₃, BeBq₂, BAlq, Znq,BAlq, ZnPBO, or ZnBTZ; a heterocyclic compound such as PBD, OXD-7, TAZ,TPBI, BPhen, or BCP; or a condensed aromatic compound such as CzPA,DPCzPA, DPPA, DNA, t-BuDNA, BANT, DPNS, DPNS2, or TPB3 can be used.

Alternatively, a high molecular compound such aspoly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation:PF-Py) orpoly[(9,9-dioctyllfluorene-2,7-diyl)-co-(2,2′-pyridine-6,6′-diyl)](abbreviation:PF-BPy) can be used. In particular, a metal complex is preferably stablewith respect to electrons.

Further, for the second organic compound contained in the seventh layer117, any of substances given below can be used. Note that although thesecond organic compound itself may emit light, in that case, theemission color of the fourth layer (light-emitting layer) 114 and theemission color of the second organic compound are preferably similarcolors in order to keep the color purity of the light-emitting element.

For example, in the case where an organic compound contained in thefourth layer 114 is an organic compound that exhibits bluish lightemission, such as YGA2S or YGAPA, the second organic compound ispreferably a compound which exhibits emission in the range of blue toblue green light, such as acridone, coumarin 102, coumarin 6H, coumarin480D, or coumarin 30.

Further, when the organic compound contained in the fourth layer(light-emitting layer) 114 is an organic compound that exhibits greenishlight emission, such as 2PCAPA, 2PCABPhA, 2DPAPA, 2DPABPhA, 2YGABPhA, orDPhAPhA, the second organic compound is preferably a compound thatexhibits light emission in the range of bluish green to yellowish green,such as N,N′-dimethylquinacridone (abbreviation: DMQd),N,N′-diphenylquinacridone (abbreviation: DPQd),9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-1),9,18-dihydro-9,18-dihydrobenzo[h]benzo[7,8]quino[2,3-b]acridine-7,16-dione(abbreviation: DMNQd-2), coumarin 30, coumarin 6, coumarin 545T, orcoumarin 153.

Alternatively, when the organic compound contained in the fourth layer(light-emitting layer) 114 is an organic compound which exhibitsyellowish light emission, such as rubrene or BPT, the second organiccompound is preferably a substance which exhibits light emission in therange of yellowish green to golden yellow, such as DMQd or(2-{2-[4-(9H-carbazol-9-yl)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCMCz).

Alternatively, when the organic compound contained in the fourth layer(light-emitting layer) 114 is an organic compound which exhibits reddishlight emission, such as p-mPhTD or p-mPhAFD, the second organic compoundis preferably a substance which exhibits light emission in the range oforange to red, such as2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),{2-(1,1-dimethylethyl)-6-[2-(2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation:DCJTB), or Nile red.

Further, when the light-emitting material contained in the fourth layer(light-emitting layer) 114 is a phosphorescent compound, the secondorganic compound is preferably also a phosphorescent compound. Forexample, when the light-emitting material is Ir(btp)₂(acac) given above,which exhibits red light emission, the second organic compound may be ared phosphorescent compound such as(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)). Note that these compounds are compoundshaving low LUMO levels among compounds that are used for light-emittingelements. Thus, by adding such a compound to the above first organiccompound, an excellent electron-trapping property can be exhibited.

For the second organic compound, among the compounds given above, aquinacridone derivative such as DMQd, DPQd, DMNQd-1, or DMNQd-2 ischemically stable and thus preferably used. That is, by applying aquinacridone derivative, the life of the light-emitting element can beparticularly extended. Further, since a quinacridone derivative exhibitsgreenish light emission, the element structure of the light-emittingelement of the present invention is particularly effective for agreenish light-emitting element. Since green is a color that needs thehighest luminance in manufacturing a full-color display, a greenishlight-emitting element deteriorates faster than light-emitting elementsof other colors in some cases. However, such a problem can be suppressedby applying the invention.

Note that as described above, the absolute value of the LUMO level ofthe second organic compound is preferably larger than the absolute valueof the LUMO level of the first organic compound by 0.3 eV or more.Therefore, the first organic compound may be selected as appropriate soas to satisfy the above-described condition depending on the kind of thesubstance used for the second organic compound.

Furthermore, the emission color of the substance having a highlight-emitting property contained in the fourth layer 114 and theemission color of the second organic compound contained in the seventhlayer 117 are preferably similar colors. Thus, a difference between thepeak value of the emission spectrum of the substance having a highlight-emitting property and the peak value of the emission spectrum ofthe second organic compound is preferably within 30 nm. The differencewithin 30 nm allows the emission color of the substance having a highlight-emitting property and the emission color of the second organiccompound to be similar colors. Therefore, if the second organic compoundemits light due to a change in voltage or the like, a change in emissioncolor can be suppressed.

Note that the second organic compound does not necessarily emit light.For example, in the case where the substance having a highlight-emitting property has higher emission efficiency, it is preferableto control the concentration of the second organic compound in theseventh layer 117 so that only light emission of the substance having ahigh light-emitting property can be obtained (to set the concentrationof the second organic compound to be slightly lower than that of thesubstance having a high light-emitting property so that light emissionof the second organic compound can be suppressed). In this case, theemission color of the substance having a high light-emitting propertyand the emission color of the second organic compound are similar colors(i.e., they have substantially the same levels of energy gaps).Therefore, energy is not easily transferred from the substance having ahigh light-emitting property to the second organic compound, and thushigh emission efficiency can be achieved.

Note that in such a case, the second organic compound is preferably acoumarin derivative such as coumarin 102, coumarin 6H, coumarin 480D,coumarin 30, coumarin 6, coumarin 545T, or coumarin 153. Because acoumarin derivative has a relatively low electron-trapping property, theconcentration of the coumarin derivative added to the first organiccompound may be relatively high. That is, the concentration can beeasily controlled, and a layer having desired properties for controllingtransport of carriers can be formed. Further, since a coumarinderivative has high emission efficiency, a decrease in efficiency of thewhole light-emitting element can be suppressed even if the secondorganic compound emits light.

Note that the seventh layer 117 in the present invention can be formedby the above-described two kinds of methods (a method for kineticallycontrolling transport of carriers and a method for thermodynamicallycontrolling transport of carriers), and the thickness of the seventhlayer 117 is preferably greater than or equal to 5 nm and less than orequal to 20 nm in either structure. This is because if the thickness islarger, an excessive decrease in rate of transport of electrons leads toan increase in driving voltage, whereas if the thickness is smaller, afunction of controlling carrier transport could be impaired.

Further, since the seventh layer 117 in the present invention is a layerfor controlling the rate of transport of electrons, the seventh layer117 may be formed between the second electrode 104 and the fourth layer(light-emitting layer) 114. More preferably, the seventh layer 117 isformed in contact with the fourth layer (light-emitting layer) 114. Bythe provision so as to be in contact with the fourth layer(light-emitting layer) 114, injection of electrons into the fourth layer(light-emitting layer) 114 can be directly controlled; thus, a change incarrier balance in the fourth layer (light-emitting layer) 114 over timecan be further suppressed, and a large effect can be obtained in termsof improvement in element life.

Note that in the case where the seventh layer 117 is formed in contactwith the fourth layer (light-emitting layer) 114, the first organiccompound contained in the seventh layer 117 and the organic compoundwhich is contained in the fourth layer (light-emitting layer) 114 inlarge amounts are preferably different organic compounds. In particular,in the case where the fourth layer (light-emitting layer) 114 contains acompound (a third organic compound) in which a substance having a highlight-emitting property is dispersed and the substance having a highlight-emitting property (a fourth organic compound), the third organiccompound and the first organic compound are preferably different organiccompounds. With such a structure, the transport of carriers (electrons)from the seventh layer 117 to the fourth layer (light-emitting layer)114 is suppressed also between the first organic compound and the thirdorganic compound, and the effect obtained by the provision of theseventh layer 117 can be further enhanced.

Further, since the seventh layer 117 contains two or more kinds ofsubstances, the carrier balance can be precisely controlled bycontrolling a combination of substances, the mixture ratio thereof, thethickness of the layer, or the like. Thus, the carrier balance can becontrolled more easily than in the conventional manner. Furthermore,since transport of carriers is controlled using the organic compound,the mixture ratio of which is smaller in the seventh layer 117, thecarrier balance does not easily change compared to the case of controlusing one substance. Accordingly, a light-emitting element which doesnot easily change over time and has a long life can be realized.

In the EL layer 103, although the fifth layer (electron-transportinglayer) 115 and the sixth layer (electron-injecting layer) 116 arestacked in that order over the above-described seventh layer (carriercontrol layer) 117, the structure thereof, a formation method, and amaterial that can be used for each layer are similar to those ofEmbodiment Mode 1. Therefore, description thereof is omitted inEmbodiment Mode 2.

Next, the second electrode 104 is formed over the sixth layer(electron-injecting layer) 116. Note that a formation method and amaterial that can be used for the second electrode 104 are also similarto those of Embodiment Mode 1. Therefore, the description thereof isomitted in Embodiment Mode 2.

Also in Embodiment Mode 2, when only the first electrode 102 is anelectrode having a light-transmitting property, light emitted from theEL layer 103 is extracted from the substrate 101 side through the firstelectrode 102, as shown in FIG. 3A. Alternatively, when only the secondelectrode 104 is an electrode having a light-transmitting property,light emitted from the EL layer 103 is extracted from the opposite sideto the substrate 101 side through the second electrode 104, as shown inFIG. 3B. Further alternatively, when the first electrode 102 and thesecond electrode 104 are both electrodes having a light-transmittingproperty, light emitted from the EL layer 103 is extracted to both thesubstrate 101 side and the opposite side, through the first electrode102 and the second electrode 104, as shown in FIG. 3C.

Note that the structure of the layers provided between the firstelectrode 102 and the second electrode 104 is not limited to the abovestructure. Note that any structure other than the above structure may beused as long as it includes at least the first layer 111 which is thehole-injecting layer, the second layer 112 which is the firsthole-transporting layer, the third layer 113 which is the secondhole-transporting layer, the fourth layer 114 which is thelight-emitting layer, and the seventh layer 117 which is the carriercontrol layer, and substances are selected so that the highest occupiedmolecular orbital level (HOMO level) of the substance used for thesecond layer 112 is deeper (the absolute value is larger) or shallower(the absolute value is smaller) than the HOMO levels of the substancesused for the first layer 111 and the third layer 113.

Alternatively, as shown in FIG. 2B, a structure in which the secondcathode 104 functioning as a cathode, the EL layer 103, and the firstelectrode 102 functioning as an anode are stacked in that order over thesubstrate 101 may be employed. Note that the EL layer 103 in this casehas a structure in which the sixth layer 116, the fifth layer 115, theseventh layer 117, the fourth layer 114, the third layer 113, the secondlayer 112, and the first layer 111 are stacked in that order over thesecond electrode 104.

Note that by use of the light-emitting element of the present invention,a passive matrix light-emitting device or an active matrixlight-emitting device in which drive of the light-emitting element iscontrolled by a thin film transistor (TFT) can be manufactured.

Note that there is no particular limitation on the structure of the TFTin the case of manufacturing an active matrix light-emitting device. Forexample, a staggered TFT or an inverted staggered TFT can be used asappropriate. Further, a driver circuit formed over a TFT substrate maybe formed of both of an n-type TFT and a p-type TFT or only either ann-type TFT or a p-type TFT. Furthermore, there is no particularlimitation on the crystallinity of a semiconductor film used for theTFT. An amorphous semiconductor film may be used, or a crystallinesemiconductor film may be used.

In the light-emitting element described in Embodiment Mode 2, the bandgap is formed by the first layer 111, the second layer 112, and thethird layer 113 which are provided between the first electrode 102 andthe fourth layer 114 which is the light-emitting layer, and thereforethe rate of transport of holes injected from the first electrode 102 canbe suppressed. Accordingly, since generation of cations in the vicinityof the fourth layer (light-emitting layer) 114 can be suppressed, adecrease in emission efficiency can be suppressed. Thus, the carrierbalance can be improved, and an element having high efficiency can beformed.

On the other hand, the seventh layer 117 is provided between the secondelectrode 104 and the fourth layer (light-emitting layer) 114 to reducethe rate of transport of carriers (electrons); accordingly, an emissionregion, which has been so far formed near the interface between thefourth layer (light-emitting layer) 114 and the third layer(hole-transporting layer) 113 because of the fast transport rate, can beformed more centrally in the fourth layer (light-emitting layer) 114than in the conventional manner.

Further, the seventh layer 117 is provided to reduce the rate oftransport of carriers (electrons); accordingly, it is possible toprevent deterioration of the third layer (hole-transporting layer) 113,which is caused by carriers (electrons) which reach, from the fourthlayer (light-emitting layer) 114, the third layer (hole-transportinglayer) 113 without contributing to light emission. Furthermore, byreducing the rate of transport of carriers (electrons), it is possiblenot only to control the amount of carriers (electrons) injected into thefourth layer (light-emitting layer) 114 but also to inhibit thecontrolled amount of carriers (electrons) from changing over time. Thus,since a decrease in the probability of recombination by deterioration ofthe balance over time can be prevented, improvement in element life(suppression of deterioration of luminance over time) can also beachieved.

In the case of the structure described in this embodiment mode, sincethe rate of transport of holes and electrons which are injected into thefourth layer 114 is controlled so as to be reduced, the carrier balancein the fourth layer 114 improves and the probability of therecombination increases; therefore, the emission efficiency can beimproved.

Note that Embodiment Mode 2 can be combined with any of the structuresdescribed in Embodiment Mode 1, as appropriate.

Embodiment Mode 3

In Embodiment Mode 3, a light-emitting element having a plurality of ELlayers of the light-emitting elements described in Embodiment Mode 1 and2 (hereinafter, referred to as a stacked-type element) is describedusing FIG. 8. This light-emitting element is a stacked-typelight-emitting element that has a plurality of EL layers (a first ELlayer 803 and a second EL layer 804) between a first electrode 801 and asecond electrode 802. Note that although a structure of two EL layers isdescribed in Embodiment Mode 3, a structure of three or more EL layersmay be employed.

In Embodiment Mode 3, the first electrode 801 functions as an anode andthe second electrode 802 functions as a cathode. Note that the firstelectrode 801 and the second electrode 802 can be made to havestructures similar to those described in Embodiment Mode 1. Further, forthe plurality of EL layers (the first EL layer 803 and the second ELlayer 804), structures similar to those described in Embodiment Mode 1and 2 can be used. Note that structures of the first EL layer 803 andthe second EL layer 804 may be the same or different from each other andcan be similar to those described in Embodiment Mode 1 or 2.

Further, a charge generation layer 805 is provided between the pluralityof EL layers (the first EL layer 803 and the second EL layer 804). Thecharge generation layer 805 has a function of injecting electrons intoone of the EL layers and injecting holes into the other of the EL layerswhen voltage is applied to the first electrode 801 and the secondelectrode 802. In Embodiment Mode 3, when voltage is applied so that thepotential of the first electrode 801 is higher than that of the secondelectrode 802, the charge generation layer 805 injects electrons intothe first EL layer 803 and injects holes into the second EL layer 804.

Note that the charge generation layer 805 preferably has alight-transmitting property in terms of light extraction efficiency.Further, the charge generation layer 805 functions even when it haslower conductivity than the first electrode 801 and the second electrode802.

The charge generation layer 805 may have either a structure in which anacceptor substance is added to a substance having a highhole-transporting property or a structure in which a donor substance isadded to a substance having a high electron-transporting property.Alternatively, both of these structures may be stacked.

In the case of the structure in which an acceptor substance is added toa substance having a high hole-transporting property, as the substancehaving a high hole-transporting property, an aromatic amine compoundsuch as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), or4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]-1,1′-biphenyl(abbreviation: BSPB) or the like can be used. The substances given heremainly are substances having a hole mobility of 10⁻⁶ cm²/(V·s) or more.However, any substance other than the above substances may be used aslong as it is a substance in which the hole-transporting property ishigher than the electron-transporting property.

Further, as examples of the substance having an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, atransition metal oxide can be given. In addition, oxides of metals thatbelong to Group 4 to Group 8 in the periodic table can be given.Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromiumoxide, molybdenum oxide, tungsten oxide, manganese oxide, and rheniumoxide are preferable since their electron-accepting properties are high.Above all, molybdenum oxide is particularly preferable because it isstable even in the atmosphere, has a low hygroscopic property, and iseasy to handle.

On the other hand, in the case of the structure in which a donorsubstance is added to a substance having a high electron-transportingproperty, as the substance having a high electron-transporting property,a metal complex having a quinoline skeleton or a benzoquinolineskeleton, such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like can be used. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: Zn(BOX)₂), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: Zn(BTZ)₂), orthe like can be used. Further alternatively, instead of the metalcomplex, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thematerials given here are mainly materials having an electron mobility of10⁻⁶ cm²/(V·s) or more. However, any substance other than the abovesubstances may be used as long as it is a substance in which theelectron-transporting property is higher than the hole-transportingproperty.

Further, for the donor material, an alkali metal, an alkaline earthmetal, a rare earth metal, a metal that belongs to Group 13 of theperiodic table, or an oxide or carbonate thereof can be used.Specifically, lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca),ytterbium (Yb), indium (In), lithium oxide, cesium carbonate, or thelike is preferably used. Alternatively, an organic compound such astetrathianaphthacene may be used for the donor material.

Note that by forming the charge generation layer 805 using any of theabove materials, an increase in driving voltage in the case where the ELlayers are stacked can be suppressed.

Although the light-emitting element having two EL layers is described inEmbodiment Mode 3, the present invention can be similarly applied to alight-emitting element in which three or more EL layers are stacked. Byarranging a plurality of EL layers to be partitioned from each otherwith a charge generation layer between a pair of electrodes, like thelight-emitting element according to Embodiment Mode 3, emission from aregion of high luminance can be realized at a low current density, andthus an element with a long life can be achieved. Further, when alighting apparatus is an application example, a drop in voltage due tothe resistance of an electrode material can be suppressed, and thusuniform emission in a large area can be achieved. Further, alight-emitting device which can be driven at low voltage and has lowpower consumption can be realized.

Further, when the EL layers have different emission colors, a desiredemission color can be obtained from the whole light-emitting element.For example, in the light-emitting element having two EL layers, when anemission color of the first EL layer and an emission color of the secondEL layer are made to be complementary colors, it is possible to obtain alight-emitting element from which white light is emitted from the wholelight-emitting element. Note that the complementary colors refer tocolors that can produce an achromatic color when they are mixed. Thatis, white light emission can be obtained by mixture of light fromsubstances whose emission colors are complementary colors.

Also in a light-emitting element having three EL layers, for example,white light can be similarly obtained from the whole light-emittingelement when an emission color of a first EL layer is red, an emissioncolor of a second EL layer is green, and an emission color of a third ELlayer is blue.

Note that Embodiment Mode 3 can be combined with any of the structuresdescribed in Embodiment Modes 1 and 2, as appropriate.

Embodiment Mode 4

In Embodiment Mode 4, a light-emitting device having a light-emittingelement of the present invention in a pixel portion is described usingFIGS. 9A and 9B. Note that FIG. 9A is a top view showing thelight-emitting device and FIG. 9B is a cross-sectional view taken alonglines A-A′ and B-B′ of FIG. 9A.

In FIG. 9A, a portion 901, indicated by a dotted line, is a drivercircuit portion (a source side driver circuit); a portion 902, indicatedby a dotted line, is a pixel portion; and a portion 903, indicated by adotted line, is a driver circuit portion (a gate side driver circuit).Further, a portion 904 is a sealing substrate; a portion 905 is asealant; and a portion enclosed by the sealant 905 is a space 907.

Note that a leading wiring 908 is a wiring for transmitting signals tobe input to the source side driver circuit 901 and the gate side drivercircuit 903 and receives signals such as a video signal, a clock signal,a start signal, and a reset signal from a flexible printed circuit (FPC)909 that is to be an external input terminal. Note that only the FPC isshown here; however, the FPC may be provided with a printed wiring board(PWB). Further, the light-emitting device in this specification includesnot only a light-emitting device itself but also a light-emitting deviceattached with an FPC or a PWB.

Next, a cross-sectional structure is described using FIG. 9B. Althoughthe driver circuit portion and the pixel portion are formed over anelement substrate 910, one pixel in the pixel portion 902 and the sourceside driver circuit 901 which is the driver circuit portion are shownhere. Note that a CMOS circuit that is a combination of an n-channel TFT923 and a p-channel TFT 924 is formed as the source side driver circuit901. Further, each driver circuit portion may be any of a variety ofcircuits such as a CMOS circuit, a PMOS circuit, and an NMOS circuit. Inthis embodiment mode, although a driver-integrated type structure inwhich a driver circuit is formed over a substrate is described, a drivercircuit is not necessarily formed over a substrate but can be formedexternally from a substrate.

Further, the pixel portion 902 is formed of a plurality of pixels eachincluding a switching TFT 911, a current control TFT 912, and a firstelectrode 913 which is electrically connected to a drain of the currentcontrol TFT 912. Note that an insulator 914 is formed to cover an endportion of the first electrode 913.

Further, the insulator 914 is desirably formed so as to have a curvedsurface with curvature at an upper end portion or a lower end portionthereof in order to make the coverage be favorable. For example, byusing positive type photosensitive acrylic as a material of theinsulator 914, the insulator 914 can be formed to have a curved surfacewith a curvature radius (greater than or equal to 0.2 μm and less thanor equal to 3 μm) only at the upper end portion. Further, either anegative type material becomes insoluble in an etchant by lightirradiation or a positive type material which becomes soluble in anetchant by light irradiation can be used as the insulator 914.

An EL layer 916 and a second electrode 917 are formed over the firstelectrode 913. Here, as a material used for the first electrode 913, anyof a variety of metals, alloys, and electroconductive compounds, or amixture thereof can be used. When the first electrode 913 is used as ananode, it is preferable to use, among these materials, any of metals,alloys, and electroconductive compounds, a mixture thereof, or the likehaving a high work function (a work function of 4.0 eV or more). Forexample, it is possible to use a single layer film of an indium tinoxide film containing silicon, an indium zinc oxide film, a titaniumnitride film, a chromium film, a tungsten film, a Zn film, a Pt film, orthe like; a stacked film of a three-layer structure of a titaniumnitride film and a film containing aluminum as its main component; athree-layer structure of a titanium nitride film, a film containingaluminum as its main component, and a titanium nitride film; or thelike. Note that when a stacked structure is employed, resistance as awiring is low, a good ohmic contact is formed, and further the firstelectrode 913 can be made to function as an anode.

Further, the EL layer 916 is formed by any of a variety of methods suchas an evaporation method using an evaporation mask, an inkjet method,and a spin coating method. The EL layer 916 has the structure describedin Embodiment Mode 1 or 2. Further, as a material used for the EL layer916, a low molecular compound or a high molecular compound (including anoligomer and a dendrimer) may be used. Further, as the material used forthe EL layer, not only an organic compound but also an inorganiccompound may be used.

Further, as a material used for the second electrode 917, any of avariety of metals, alloys, and electroconductive compounds, or a mixturethereof can be used. When the second electrode 917 is used as a cathode,it is preferable to use, among these materials, any of metals, alloys,and electroconductive compounds, a mixture thereof, or the like having alow work function (a work function of 3.8 eV or less). For example,elements belonging to Group 1 and 2 of the periodic table, that is,alkali metals such a lithium (Li) and cesium (Cs) and alkaline earthmetals such as magnesium (Mg), calcium (Ca), and strontium (Sr); alloysthereof (MgAg or AlLi); and the like are given.

Note that in the case where light generated in the EL layer 916 istransmitted through the second electrode 917, for the second electrode917, a stack of a metal thin film with a reduced thickness and atransparent conductive film (indium tin oxide (ITO), indium tin oxidecontaining silicon or silicon oxide, indium zinc oxide (IZO), or indiumoxide containing tungsten oxide and zinc oxide (IWZO)) can also be used.

Furthermore, a structure is provided in which the sealing substrate 904is attached using the sealant 905 to the element substrate 910 so thatthe light-emitting element 918 is provided in the space 907 surroundedby the element substrate 910, the sealing substrate 904, and the sealant905. Note that the space 907 is filled with a filler. There are caseswhere the space 907 is filled with an inert gas (nitrogen, argon, or thelike), and where the space 907 may be filled with the sealant 905.

Note that an epoxy-based resin is preferably used for the sealant 905.Further, it is preferable that these materials hardly transmit water oroxygen. Further, as the sealing substrate 904, instead of a glasssubstrate or a quartz substrate, a plastic substrate formed offiberglass-reinforced plastic (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used.

As described above, an active matrix light-emitting device having thelight-emitting element of the present invention can be obtained.

Further, the light-emitting element of the present invention can be usedfor a passive matrix light-emitting device instead of the above activematrix light-emitting device. FIGS. 10A and 10B are a perspective viewand a cross-sectional view of a passive matrix light-emitting deviceusing the light-emitting element of the present invention. Note thatFIG. 10A is a perspective view of the light-emitting device and FIG. 10Bis a cross-sectional view taken along a line X-Y of FIG. 10A.

In FIGS. 10A and 10B, an EL layer 1004 is provided between a firstelectrode 1002 and a second electrode 1003 over a substrate 1001. An endportion of the first electrode 1002 is covered by an insulating layer1005. In addition, a partition layer 1006 is provided over theinsulating layer 1005. Sidewalls of the partition layer 1006 have aslant such that a distance between one sidewall and the other sidewallbecomes narrower as the sidewalls gets closer to a surface of thesubstrate. In other words, a cross section taken in the direction of ashorter side of the partition layer 1006 has a trapezoidal shape, andthe base of the trapezoid (a side of the trapezoid which is parallel tothe surface of the insulating layer 1005 and is in contact with theinsulating layer 1005) is shorter than the upper side of the trapezoid(a side of the trapezoid which is parallel to the surface of theinsulating layer 1005 and is not in contact with the insulating layer1005). The provision of the partition layer 1006 in this manner canprevent the light-emitting element from being defective due to staticelectricity or the like.

Accordingly, the passive matrix light-emitting device using thelight-emitting element of the present invention can be obtained.

Note that any of the light-emitting devices described in this embodimentmode (the active matrix light-emitting device and the passive matrixlight-emitting device) are formed using the light-emitting element ofthe present invention, which has high emission efficiency, andaccordingly a light-emitting device having reduced power consumption canbe obtained.

Note that Embodiment Mode 4 can be combined with any of the structuresdescribed in Embodiment Modes 1 to 3, as appropriate.

Embodiment Mode 5

In Embodiment Mode 5, an electronic appliance including, as a partthereof, the light-emitting device of the present invention, which isdescribed in Embodiment Mode 4, is described. Examples of the electronicappliance include cameras such as video cameras or digital cameras,goggle type displays, navigation systems, audio playback devices (e.g.,car audio systems and audio systems), computers, game machines, portableinformation terminals (e.g., mobile computers, cellular phones, portablegame machines, and electronic books), image playback devices in which arecording medium is provided (specifically, devices that are capable ofplaying back recording media such as digital versatile discs (DVDs) andequipped with a display unit that can display images), and the like.Specific examples of these electronic appliances are shown in FIGS. 11Ato 11D.

FIG. 11A shows a television set according to the present invention,which includes a housing 9101, a support base 9102, a display portion9103, a speaker portion 9104, a video input terminal 9105, and the like.In this television set, the light-emitting device of the presentinvention can be applied to the display portion 9103. Since thelight-emitting device of the present invention has a feature of highemission efficiency, a television set having reduced power consumptioncan be obtained by applying the light-emitting device of the presentinvention.

FIG. 11B shows a computer according to the present invention, whichincludes a main body 9201, a housing 9202, a display portion 9203, akeyboard 9204, an external connection port 9205, a pointing device 9206,and the like. In this computer, the light-emitting device of the presentinvention can be applied to the display portion 9203. Since thelight-emitting device of the present invention has a feature of highemission efficiency, a computer having reduced power consumption can beobtained by applying the light-emitting device of the present invention.

FIG. 11C shows a cellular phone according to the present invention,which includes a main body 9401, a housing 9402, a display portion 9403,an audio input portion 9404, an audio output portion 9405, operationkeys 9406, an external connection port 9407, an antenna 9408, and thelike. In this cellular phone, the light-emitting device of the presentinvention can be applied to the display portion 9403. Since thelight-emitting device of the present invention has a feature of highemission efficiency, a cellular phone having reduced power consumptioncan be obtained by applying the light-emitting device of the presentinvention.

FIG. 11D shows a camera according to the present invention, whichincludes a main body 9501, a display portion 9502, a housing 9503, anexternal connection port 9504, a remote control receiver 9505, an imagereceiver 9506, a battery 9507, an audio input portion 9508, operationkeys 9509, an eye piece portion 9510, and the like. In this camera, thelight-emitting device of the present invention can be applied to thedisplay portion 9502. Since the light-emitting device of the presentinvention has a feature of high emission efficiency, a camera havingreduced power consumption can be obtained by applying the light-emittingdevice of the present invention.

As described above, the applicable range of the light-emitting device ofthe present invention is wide so that this light-emitting device can beapplied to electronic appliances of a variety of fields. By applying thelight-emitting device of the present invention, an electronic appliancehaving reduced power consumption can be obtained.

Further, the light-emitting device of the present invention can also beused as a lighting apparatus. FIG. 12 shows an example of a liquidcrystal display device using the light-emitting device of the presentinvention as a backlight. The liquid crystal display device shown inFIG. 12 includes a housing 1201, a liquid crystal layer 1202, abacklight 1203, and a housing 1204. The liquid crystal layer 1202 isconnected to a driver IC 1205. Further, the light-emitting device of thepresent invention is used as the backlight 1203 to which current issupplied through a terminal 1206.

By use of the light-emitting device of the present invention as abacklight of a liquid crystal display device as described above, abacklight having low power consumption can be obtained. Further, sincethe light-emitting device of the present invention is a surface emittinglighting apparatus and can be formed to have a large area, a larger-areabacklight can also be obtained. Accordingly, a larger-area liquidcrystal display device having low power consumption can be obtained.

FIG. 13 shows an example in which the light-emitting device to which thepresent invention is applied is used as a desk lamp that is a lightingapparatus. The desk lamp shown in FIG. 13 includes a housing 1301 and alight source 1302, and the light-emitting device of the presentinvention is used as the light source 1302. The light-emitting device ofthe present invention has the light-emitting element having highemission efficiency and therefore can be used as a desk lamp having lowpower consumption.

FIG. 14 shows an example in which the light-emitting device to which thepresent invention is applied is used as an indoor lighting apparatus3001. The light-emitting device of the present invention can be formedto have a large area and therefore can be used as a large-area lightingdevice. Further, the light-emitting device of the invention has thelight-emitting element having high emission efficiency and therefore canbe used as a lighting apparatus having low power consumption. Thus, in aroom where a light-emitting device to which the present invention isapplied is used as the indoor lighting device 1401, a television set1402 according to the present invention, as described using FIG. 11A, isplaced; then, public broadcasting and movies can be watched.

Note that Embodiment Mode 5 can be combined with any of the structuresdescribed in Embodiment Modes 1 to 5, as appropriate.

EXAMPLE 1

In Example 1, fabrication methods of light-emitting elements having thestructure described in Embodiment Mode 1, which are light-emittingelements of the present invention, and measurement results of theelement characteristics thereof are described. Note that the elementstructure of the light-emitting elements described in this example(light-emitting elements 1 to 3) are shown in FIG. 15A and the elementstructure of a light-emitting element to be compared to theselight-emitting elements is shown in FIG. 15B. Further, structuralformulae of organic compounds used in Example 1 are shown below.

(Fabrication of Light-Emitting Element 1)

The light-emitting element 1 is a light-emitting element having thestructure described using FIG. 1A in Embodiment Mode 1. Specifically,the light-emitting element 1 is a light-emitting element of the casewhere the HOMO level of a second layer 1512 of FIG. 15A is deeper (theabsolute value is larger) than the HOMO levels of a first layer 1511 anda third layer 1513.

First, indium tin oxide containing silicon oxide was deposited over aglass substrate 1501 by a sputtering method to form a first electrode1502. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

Next, an EL layer 1503 in which a plurality of layers is stacked overthe first electrode 1502 is formed. In this example, the EL layer 1503has a structure in which the first layer 1511 which is thehole-injecting layer, a second layer 1512 which is the hole-transportinglayer, the third layer 1513 which is the hole-transporting layer, afourth layer 1514 which is the light-emitting layer, a fifth layer 1515which is the electron-transporting layer, and a sixth layer 1516 whichis the electron-injecting layer are stacked in that order.

The substrate provided with the first electrode 1502 was fixed on asubstrate holder that was provided in a vacuum evaporation apparatus sothat a surface provided with the first electrode 1502 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1502,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1511which is the hole-injecting layer. The thickness thereof was set to be30 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next,N-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF) was deposited over the first layer 1511 to athickness of 10 nm by an evaporation method using resistive heating toform the second layer 1512 which is the hole-transporting layer.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited over the second layer 1512 to a thickness of 20 nm by anevaporation method using resistive heating to form the third layer 1513which is the hole-transporting layer.

Next, the fourth layer 1514 which is the light-emitting layer is formedover the third layer 1513. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S), the fourth layer 1514 with a thickness of 30 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to YGA2S was adjusted to be 1:0.04 (=CzPA:YGA2S).

Furthermore, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) wasdeposited over the fourth layer 1514 to a thickness of 20 nm, andbathophenanthroline (abbreviation: BPhen) was deposited thereover to athickness of 10 nm by an evaporation method using resistive heating toform the fifth layer 1515 that is the electron-transporting layer.

Lithium fluoride (LiF) was deposited over the fifth layer 1515 to athickness of 1 nm to form the sixth layer 1516 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form a second electrode1504. Accordingly, the light-emitting element 1 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 1 obtained as described above was not exposedto the atmosphere, and then operation characteristics of thislight-emitting element were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 1 are shown in FIG. 17. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 18. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 19. In addition,the emission spectrum at current of 1 mA is shown in FIG. 20.

The CIE chromaticity coordinate of the light-emitting element 1 at aluminance of 1000 cd/m² was (x=0.16, y=0.18), and blue light whichderives from YGA2S was emitted. In addition, at a luminance of 1000cd/m², the current efficiency was 5.4 cd/A, meaning that high efficiencywas exhibited. At a luminance of 1000 cd/m², the driving voltage was 5.6V.

(Fabrication of Light-Emitting Element 2)

The light-emitting element 2 is, similarly to the light-emitting element1, a light-emitting element having the structure described using FIG. 1Ain Embodiment Mode 1. Specifically, the light-emitting element 2 is alight-emitting element of the case where the HOMO level of the secondlayer 1512 of FIG. 15A is deeper (the absolute value is larger) than theHOMO levels of the first layer 1511 and the third layer 1513.

The light-emitting element 2 was fabricated in a similar manner to thelight-emitting element 1 except that4,4′,4″-tris(N-carbazolyl)triphenylamine (abbreviation: TCTA) was usedinstead of YGASF used for the second layer 1512 of the light-emittingelement 1.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 2 was not exposed to the atmosphere,and then operation characteristics of this light-emitting element weremeasured. Note that the measurements were performed at room temperature(in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 2 are shown in FIG. 17. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 18. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 19. In addition,the emission spectrum at current of 1 mA is shown in FIG. 20.

The CIE chromaticity coordinate of the light-emitting element 2 at aluminance of 1000 cd/m² was (x=0.16, y=0.18), and blue light whichderives from YGA2S was emitted. In addition, at a luminance of 1000cd/m², the current efficiency was 7.7 cd/A, meaning that high efficiencywas exhibited. At a luminance of 1000 cd/m², the driving voltage was 6.4V.

(Fabrication of Light-Emitting Element 3)

Unlike the light-emitting element 1 or the light-emitting element 2, thelight-emitting element 3 is a light-emitting element having thestructure described using FIG. 1B in Embodiment Mode 1. Specifically,the light-emitting element 3 is a light-emitting element of the casewhere the HOMO level of the second layer 1512 of FIG. 15A is shallower(the absolute value is smaller) than the HOMO levels of the first layer1511 and the third layer 1513.

The light-emitting element 3 was fabricated in a similar manner to thelight-emitting element 1, using4,4′-bis(N-{4-[N′-(3-methylphenyl)-N′-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD) instead of YGASF used for the second layer 1512 ofthe light-emitting element 1.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 3 was not exposed to the atmosphere,and then operation characteristics of this light-emitting element weremeasured. Note that the measurements were performed at room temperature(in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 3 are shown in FIG. 17. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 18. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 19. In addition,the emission spectrum at current of 1 mA is shown in FIG. 20.

The CIE chromaticity coordinate of the light-emitting element 3 at aluminance of 1000 cd/m² was (x=0.1, y=0.17), and blue light whichderives from YGA2S was emitted. In addition, at a luminance of 1000cd/m², the current efficiency was 5.1 cd/A, meaning that high efficiencywas exhibited. At a luminance of 1000 cd/m², the driving voltage was 5.2V.

(Fabrication of Light-Emitting Element 4)

Next, as a light-emitting element for comparison, a light-emittingelement 4 having a structure shown in FIG. 15B (a structure in which thesecond layer 1512 of the above light-emitting elements 1 to 3 is notprovided) is fabricated. The fabrication method is described below.

First, indium tin oxide containing silicon oxide was deposited over theglass substrate 1501 by a sputtering method to form the first electrode1502. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

The substrate provided with the first electrode 1502 was fixed on asubstrate holder that was provided in a vacuum evaporation apparatus sothat a surface provided with the first electrode 1502 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1502,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1511which is the hole-injecting layer. The thickness thereof was set to be30 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited to a thickness of 30 nm by an evaporation method usingresistive heating to form the third layer 1513 which is thehole-transporting layer.

Next, the fourth layer 1514 which is the light-emitting layer is formedover the third layer 1513. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S), the fourth layer 1514 with a thickness of 30 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to YGA2S was adjusted to be 1:0.04 (=CzPA:YGA2S).

Then, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) wasdeposited over the fourth layer 1514 to a thickness of 20 nm, andbathophenanthroline (abbreviation: BPhen) was deposited thereover to athickness of 10 nm by an evaporation method using resistive heating toform the fifth layer 1515 that is the electron-transporting layer.

Next, lithium fluoride (LiF) was deposited over the fifth layer 1515 toa thickness of 1 nm to form the sixth layer 1516 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form the second electrode1504. Accordingly, the light-emitting element 4 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 4 obtained as described above was not exposedto the atmosphere, and then operation characteristics of thelight-emitting element 4 were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 4 are shown in FIG. 17. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 18. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 19.

The CIE chromaticity coordinate of the light-emitting element 4 at aluminance of 1000 cd/m² was (x=0.16, y=0.17), and blue light whichderives from YGA2S was emitted similarly to the light-emitting elements1 to 3. Further, the current efficiency of the light-emitting element 4was 3.6 cd/A, and thus it is found that the current efficiency was lowerthan those of the light-emitting elements 1 to 3.

As described above, it can be seen that the light-emitting elements 1 to3 have higher efficiency than the light-emitting element 4. Thus, it isunderstood that a light-emitting element having high efficiency can beobtained by applying the present invention.

EXAMPLE 2

In Example 2, fabrication methods of light-emitting elements having thestructure described in Embodiment Mode 2, which are light-emittingelements of the present invention, and measurement results of theelement characteristics thereof are described. Note that the elementstructure of the light-emitting elements described in this example(light-emitting elements 5 to 7) are shown in FIG. 16A and the elementstructure of a light-emitting element 8 to be compared to theselight-emitting elements is shown in FIG. 16B. Further, structuralformulae of organic compounds used in Example 2 are shown below. Notethat the organic compounds described in Example 1 can be referred toExample 1 and the description thereof is omitted here.

(Fabrication of Light-Emitting Element 5)

The light-emitting element 5 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 5 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of a first layer 1611 anda third layer 1613. Furthermore, the light-emitting element 5 is alight-emitting element of the case where the seventh layer kineticallycontrols carriers (electrons) as illustrated in the conceptual diagramsof FIGS. 6A and 6B.

First, indium tin oxide containing silicon oxide was deposited over aglass substrate 1601 by a sputtering method to form a first electrode1602. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

Next, an EL layer 1603 in which a plurality of layers is stacked overthe first electrode 1602 is formed. In this example, the EL layer 1603has a structure in which the first layer 1611 which is thehole-injecting layer, a second layer 1612 which is the hole-transportinglayer, the third layer 1613 which is the hole-transporting layer, afourth layer 1614 which is the light-emitting layer, a seventh layer1617 which is the carrier control layer for controlling transport ofelectron carriers, a fifth layer 1615 which is the electron-transportinglayer, and a sixth layer 1616 which is the electron-injecting layer arestacked in that order.

The substrate provided with the first electrode 1602 was fixed on asubstrate holder that was provided in a vacuum evaporation apparatus sothat a surface provided with the first electrode 1602 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1602,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1611which is the hole-injecting layer. The thickness thereof was set to be30 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next,N-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF) was deposited over the first layer 1611 to athickness of 10 nm by an evaporation method using resistive heating toform the second layer 1612 that is the hole-transporting layer.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited over the second layer 1612 to a thickness of 20 nm by anevaporation method using resistive heating to form the third layer 1613that is the hole-transporting layer.

Next, the fourth layer 1614 which is the light-emitting layer is formedover the third layer 1613. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), the fourth layer 1614 with a thickness of 30 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to 2PCAPA could be 1:0.05 (=CzPA:2PCAPA).

Furthermore, over the fourth layer 1614,tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) were co-evaporated to form the seventh layer 1617which is the carrier control layer for controlling electron carrierswith a thickness of 10 nm. Here, the evaporation rate was adjusted suchthat the weight ratio of Alq to 2PCAPA could be 1:0.1 (=Alq:2PCAPA).

Then, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) wasdeposited over the seventh layer 1617 to a thickness of 30 nm by anevaporation method using resistive heating to form the fifth layer 1615that is the electron-transporting layer.

Lithium fluoride (LiF) was deposited over the fifth layer 1615 to athickness of 1 nm to form the sixth layer 1616 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form a second electrode1604. Accordingly, the light-emitting element 5 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 5 of the present invention which is obtainedas described above was not exposed to the atmosphere, and then operationcharacteristics of this light-emitting element were measured. Note thatthe measurements were performed at room temperature (in an atmospherekept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 5 are shown in FIG. 21. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 22. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 23. In addition,the emission spectrum at current of 1 mA is shown in FIG. 24. Further,FIG. 25 shows the results of continuous lighting tests in which thelight-emitting element 5 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² is100%).

The CIE chromaticity coordinate of the light-emitting element 5 at aluminance of 5000 cd/m² was (x=0.29, y=0.63), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 19 cd/A, meaning that high efficiencywas exhibited. At a luminance of 5000 cd/m², the driving voltage was 8.4V.

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 5 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 81%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 5 has a long life in additionto high efficiency.

(Fabrication of Light-Emitting Element 6)

The light-emitting element 6 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 6 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of the first layer 1611and the third layer 1613. Furthermore, the light-emitting element 6 is alight-emitting element of the case where the seventh layerthermodynamically controls carriers (electrons) as illustrated in theconceptual diagram of FIG. 7.

The light-emitting element 6 was fabricated in a similar manner to thelight-emitting element 5 except that a co-evaporated film of Alq andN,N′-diphenylquinacridone (abbreviation: DPQd) was used instead of aco-evaporated film of Alq and 2PCAPA, which was used for the seventhlayer 1617 of the light-emitting element 5. Here, the evaporation ratewas adjusted such that the weight ratio of Alq to DPQd could be 1:0.005(=Alq:DPQd).

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 6 of the present invention was notexposed to the atmosphere, and then operation characteristics of thislight-emitting element were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 6 are shown in FIG. 21. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 22. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 23. In addition,the emission spectrum at current of 1 mA is shown in FIG. 24. Further,FIG. 25 shows the results of continuous lighting tests in which thelight-emitting element 6 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 6 at aluminance of 5000 cd/m² was (x=0.29, y=0.62), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 15 cd/A, meaning that high efficiencywas exhibited. At a luminance of 5000 cd/m², the driving voltage was 9V.

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 6 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 80%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 6 has a long life in additionto high efficiency.

(Fabrication of Light-Emitting Element 7)

The light-emitting element 7 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 6 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of the first layer 1611and the third layer 1613. Furthermore, the light-emitting element 7 is alight-emitting element of the case where the seventh layer kineticallycontrols carriers (electrons) as illustrated in the conceptual diagramof FIGS. 6A and 6B.

The light-emitting element 7 was fabricated in a similar manner to thelight-emitting element 5, usingN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylvinyl-4,4′-diamine(abbreviation: YGABP) instead of YGASF used for the fifth layer 1612 ofthe light-emitting element 5.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 7 of the present invention was notexposed to the atmosphere, and then operation characteristics of thislight-emitting element were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 7 are shown in FIG. 21. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 22. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 23. In addition,the emission spectrum at current of 1 mA is shown in FIG. 24. Further,FIG. 25 shows the results of continuous lighting tests in which thelight-emitting element 7 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 7 at aluminance of 5000 cd/m² was (x=0.29, y=0.63), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 17 cd/A, meaning that high efficiencywas exhibited. At a luminance of 5000 cd/m², the driving voltage was 8.1V.

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 7 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 85%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 7 has a long life in additionto high efficiency.

(Fabrication of Light-Emitting Element 8)

Next, as a light-emitting element for comparison, a light-emittingelement 8 having a structure shown in FIG. 16B (a structure in which thesecond layer 1612 and the seventh layer 1617 of the above light-emittingelements 5 to 7 are not provided) is fabricated. The fabrication methodis described below.

First, indium tin oxide containing silicon oxide was deposited over theglass substrate 1601 by a sputtering method to form the first electrode1602. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

Next, the substrate provided with the first electrode 1602 was fixed ona substrate holder that was provided in a vacuum evaporation apparatusso that a surface provided with the first electrode 1602 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1602,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1611which is the hole-injecting layer. The thickness thereof was set to be50 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited to a thickness of 10 nm by an evaporation method usingresistive heating to form the third layer 1613 that is thehole-transporting layer.

Next, the fourth layer 1614 which is the light-emitting layer is formedover the third layer 1613. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), the fourth layer 1614 with a thickness of 40 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to 2PCAPPA could be 1:0.05 (=CzPA:2PCAPA).

Then, tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) wasdeposited over the fourth layer 1614 to a thickness of 30 nm by anevaporation method using resistive heating to form the fifth layer 1615that is the electron-transporting layer.

Next, lithium fluoride (LiF) was deposited over the fifth layer 1615 toa thickness of 1 nm to form the sixth layer 1616 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form the second electrode1604. Accordingly, the light-emitting element 8 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 8 obtained as described above was not exposedto the atmosphere, and then operation characteristics of thelight-emitting element 8 were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 8 are shown in FIG. 21. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 22. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 23. Further, FIG.25 shows the results of continuous lighting tests in which thelight-emitting element 8 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 8 at aluminance of 5000 cd/m² was (x=0.30, y=0.62), the current efficiency was13 cd/A, and green light which derives from 2PCAPA was emitted similarlyto the light-emitting elements 5 to 7. It is found that the currentefficiency was lower than those of the light-emitting elements 5 to 7.Further, the continuous lighting tests were conducted in which thelight-emitting element 8 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, theluminance has decreased to 85% of the initial luminance after 220 hours,as shown in FIG. 25. Thus, the light-emitting element 8 exhibited a lifeshorter than the light-emitting elements 5 to 7.

As described above, it can be seen that the light-emitting elements 5 to7 have higher efficiency than the light-emitting element 8 and furtherhas a long life. Thus, it is understood that a light-emitting elementhaving high efficiency and a long life can be obtained by applying thepresent invention.

EXAMPLE 3

In Example 3, fabrication methods of light-emitting elements(light-emitting elements 9 to 11) having a structure that is the same asthe element structure described in Example 2 and measurement results ofthe element characteristics thereof are described. Note that thisexample is also described with reference to FIGS. 16A and 16B. Further,organic compounds used in Example 3 are referred to Example 1 or 2 andthe description thereof is omitted here.

(Fabrication of Light-Emitting Element 9)

The light-emitting element 9 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 9 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of the first layer 1611and the third layer 1613. Furthermore, the light-emitting element 9 is alight-emitting element of the case where the seventh layer kineticallycontrols carriers (electrons) as illustrated in the conceptual diagramsof FIGS. 6A and 6B.

First, indium tin oxide containing silicon oxide was deposited over theglass substrate 1601 by a sputtering method to form the first electrode1602. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

Next, the EL layer 1603 in which a plurality of layers is stacked overthe first electrode 1602 is formed. In this example, the EL layer 1603has a structure in which the first layer 1611 which is thehole-injecting layer, the second layer 1612 which is thehole-transporting layer, the third layer 1613 which is thehole-transporting layer, the fourth layer 1614 which is thelight-emitting layer, the seventh layer 1617 which is a carrier controllayer for controlling transport of electron carriers, the fifth layer1615 which is the electron-transporting layer, and the sixth layer 1616which is the electron-injecting layer are stacked in that order.

The substrate provided with the first electrode 1602 was fixed on asubstrate holder that was provided in a vacuum evaporation apparatus sothat a surface provided with the first electrode 1602 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1602,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1611which is the hole-injecting layer. The thickness thereof was set to be30 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next,N-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF) was deposited over the first layer 1611 to athickness of 10 nm by an evaporation method using resistive heating toform the second layer 1612 that is the hole-transporting layer.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited over the second layer 1612 to a thickness of 20 nm by anevaporation method using resistive heating to form the third layer 1613that is the hole-transporting layer.

Next, the fourth layer 1614 which is the light-emitting layer is formedover the third layer 1613. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), the fourth layer 1614 with a thickness of 30 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to 2PCAPA could be 1:0.05 (=CzPA:2PCAPA).

Furthermore, over the fourth layer 1614,tris(8-quinolinolato)aluminum(III) (abbreviation: Alq) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA) were co-evaporated to form the seventh layer 1617which is the carrier control layer for controlling electron carrierswith a thickness of 10 nm. Here, the evaporation rate was adjusted suchthat the weight ratio of Alq to 2PCAPA could be 1:0.1 (=Alq:2PCAPA).

Then, bathophenanthroline (abbreviation: BPhen) was deposited over theseventh layer 1617 to a thickness of 30 nm by an evaporation methodusing resistive heating to form the fifth layer 1615 that is theelectron-transporting layer.

Lithium fluoride (LiF) was deposited over the fifth layer 1615 to athickness of 1 nm to form the sixth layer 1616 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form the second electrode1604. Accordingly, the light-emitting element 9 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 9 of the present invention which is obtainedas described above was not exposed to the atmosphere, and then operationcharacteristics of this light-emitting element were measured. Note thatthe measurements were performed at room temperature (in an atmospherekept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 9 are shown in FIG. 26. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 27. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 28. In addition,the emission spectrum at current of 1 mA is shown in FIG. 29. Further,FIG. 30 shows the results of continuous lighting tests in which thelight-emitting element 9 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 9 at aluminance of 5000 cd/m² was (x=0.29, y=0.63), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 24 cd/A, meaning that extremely highefficiency was exhibited. At a luminance of 5000 cd/m², the drivingvoltage was 5.4 V.

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 9 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 71%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 9 has a long life in additionto high efficiency.

(Fabrication of Light-Emitting Element 10)

The light-emitting element 10 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 10 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of the first layer 1611and the third layer 1613. Furthermore, the light-emitting element 10 isa light-emitting element of the case where the seventh layerthermodynamically controls carriers (electrons) as illustrated in theconceptual diagram of FIG. 7.

The light-emitting element 10 was fabricated in a similar manner to thelight-emitting element 9 except that a co-evaporated film of Alq and N,NA-diphenylquinacridone (abbreviation: DPQd) was used instead of aco-evaporated film of Alq and 2PCAPA which was used for the seventhlayer 1617 of the light-emitting element 9. Here, the evaporation ratewas adjusted such that the weight ratio of Alq to DPQd could be 1:0.005(=Alq:DPQd).

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 10 of the present invention was notexposed to the atmosphere, and then operation characteristics of thislight-emitting element were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 10 are shown in FIG. 26. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 27. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 28. In addition,the emission spectrum at current of 1 mA is shown in FIG. 29. Further,FIG. 30 shows the results of continuous lighting tests in which thelight-emitting element 10 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 10 at aluminance of 5000 cd/m² was (x=0.28, y=0.62), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 19 cd/A, meaning that extremely highefficiency was exhibited. At a luminance of 5000 cd/m², the drivingvoltage was 6.4 V

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 10 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 80%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 10 has a long life inaddition to high efficiency.

(Fabrication of Light-Emitting Element 11)

The light-emitting element 11 is a light-emitting element having thestructure described using FIG. 4A in Embodiment Mode 2. Specifically,the light-emitting element 11 is a light-emitting element of the casewhere the HOMO level of the second layer 1612 of FIG. 16A is deeper (theabsolute value is larger) than the HOMO levels of the first layer 1611and the third layer 1613. By co-evaporation of Furthermore, thelight-emitting element 11 is a light-emitting element of the case wherethe seventh layer kinetically controls carriers (electrons) asillustrated in the conceptual diagrams of FIGS. 6A and 6B.

The light-emitting element 11 was fabricated in a similar manner to thelight-emitting element 9 except that YGASF which was used for the secondlayer 1612 of the light-emitting element 9 was replaced byN,N′-bis[4-(9H-carbazol-9-yl)phenyl-N,N′-diphenylvinyl-4,4′-diamine(abbreviation: YGABP).

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe obtained light-emitting element 11 of the present invention was notexposed to the atmosphere, and then operation characteristics of thislight-emitting element were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 11 are shown in FIG. 26. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 27. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 28. In addition,the emission spectrum at current of 1 mA is shown in FIG. 29. Further,FIG. 30 shows the results of continuous lighting tests in which thelight-emitting element 11 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 11 at aluminance of 5000 cd/m² was (x=0.29, y=0.63), and green light whichderives from 2PCAPA was emitted. In addition, at a luminance of 5000cd/m², the current efficiency was 22 cd/A, meaning that extremely highefficiency was exhibited. At a luminance of 5000 cd/m², the drivingvoltage was 5.2 V

Furthermore, the continuous lighting tests were conducted in which thelight-emitting element 11 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, 77%of the initial luminance was maintained even after 1000 hours. Thus, itwas proved that the light-emitting element 11 has a long life inaddition to high efficiency.

(Fabrication of Light-Emitting Element 12)

Next, as a light-emitting element for comparison, the light-emittingelement 12 having a structure shown in FIG. 16B (a structure in whichthe second layer 1612 and the seventh layer 1617 of the abovelight-emitting elements 9 to 11 are not provided) is fabricated. Thefabrication method is described below.

First, indium tin oxide containing silicon oxide was deposited over theglass substrate 1601 by a sputtering method to form the first electrode1602. Note that the thickness thereof was set to be 110 nm and the areathereof was set to be 2 mm×2 mm.

Next, the substrate provided with the first electrode 1602 was fixed ona substrate holder that was provided in a vacuum evaporation apparatusso that a surface provided with the first electrode 1602 faced downward.The pressure in the vacuum evaporation apparatus was reduced toapproximately 10⁻⁴ Pa. Then, over the first electrode 1602,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB) andmolybdenum(VI) oxide were co-evaporated to form the first layer 1611which is the hole-injecting layer. The thickness thereof was set to be50 nm, and the evaporation rate was adjusted such that the weight ratioof NPB to molybdenum(VI) oxide was adjusted to be 4:1 (=NPB:molybdenumoxide). Note that a co-evaporation method is an evaporation method inwhich evaporation is performed using a plurality of evaporation sourcesin one treatment chamber at the same time.

Next, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB)was deposited to a thickness of 10 nm by an evaporation method usingresistive heating to form the third layer 1613 that is thehole-transporting layer.

Next, the fourth layer 1614 which is the light-emitting layer is formedover the third layer 1613. By co-evaporation of9-[4-(N-carbazolyl)phenyl]-10-phenylanthracene (abbreviation: CzPA) andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), the fourth layer 1614 with a thickness of 40 nmwas formed. Here, the evaporation rate was adjusted such that the weightratio of CzPA to 2PCAPPA could be 1:0.05 (=CzPA:2PCAPA).

Then, bathophenanthroline (abbreviation: BPhen) was deposited over thefourth layer 1614 to a thickness of 30 nm by an evaporation method usingresistive heating to form the fifth layer 1615 that is theelectron-transporting layer.

Next, lithium fluoride (LiF) was deposited over the fifth layer 1615 toa thickness of 1 nm to form the sixth layer 1616 that is theelectron-injecting layer.

Lastly, aluminum was deposited to a thickness of 200 nm by anevaporation method using resistive heating to form the second electrode1604. Accordingly, the light-emitting element 12 was fabricated.

Sealing was performed in a glove box under a nitrogen atmosphere so thatthe light-emitting element 12 obtained as described above was notexposed to the atmosphere, and then operation characteristics of thislight-emitting element 12 were measured. Note that the measurements wereperformed at room temperature (in an atmosphere kept at 25° C.).

The current density vs. luminance characteristics of the light-emittingelement 12 are shown in FIG. 26. In addition, the voltage vs. luminancecharacteristics are shown in FIG. 27. In addition, the luminance vs.current efficiency characteristics are shown in FIG. 28. Further, FIG.30 shows the results of continuous lighting tests in which thelight-emitting element 12 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m² (the vertical axisindicates the relative luminance on the assumption that 5000 cd/m² isassumed as 100%).

The CIE chromaticity coordinate of the light-emitting element 12 at aluminance of 5000 cd/m² was (x=0.30, y=0.62), the current efficiency was17 cd/A, and green light which derives from 2PCAPA was emitted similarlyto the light-emitting elements 9 to 11. It is found that the currentefficiency was lower than those of the light-emitting elements 9 to 11.Further, the continuous lighting tests were conducted in which thelight-emitting element 12 was continuously lit by constant currentdriving with the initial luminance set at 5000 cd/m². As a result, theluminance has decreased to 70% of the initial luminance after 180 hours,as shown in FIG. 30. Thus, the light-emitting element 8 exhibited a lifeshorter than the light-emitting elements 9 to 11.

As described above, it can be seen that the light-emitting elements 9 to11 have higher efficiency than the light-emitting element 12 and furtherhas a long life. Thus, it is understood that a light-emitting elementhaving high efficiency and a long life can be obtained by applying thepresent invention.

EXAMPLE 4

In this example, the oxidation characteristics ofN-[4-(9H-carbazol-9-yl)phenyl]-N-phenyl-spiro-9,9′-bifluoren-2-amine(abbreviation: YGASF),N,N′-bis[4-(9H-carbazol-9-yl)phenyl-N,N′-diphenylvinyl-4,4′-diamine(abbreviation: YGABP), 4,4′,4″-tris(N-carbazolyl)triphenylamine(abbreviation: TCTA), and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB), which were used for the second layers (the secondlayer 1512 of FIG. 15A and the second layer 1612 of FIG. 16A) and thethird layers (the third layer 1513 of FIGS. 15A and 15B and the thirdlayer 1613 of FIGS. 16A and 16B) which were the hole-transporting layersof the light-emitting elements fabricated in Example 1 to 3 (thelight-emitting elements 1 to 3, the light-emitting elements 5 to 7, andthe light-emitting elements 9 to 11) were evaluated by cyclicvoltammetry (CV) measurements.

Further, from the measurements, the LUMO levels of YGASF, YGABP, TCTA,and NPB were obtained. Note that an electrochemical analyzer (ALS model600A or 600C, manufactured by BAS Inc.) was used for the measurements.

As for a solution used for the CV measurements, dehydrateddimethylformamide (DMF) (product of Sigma-Aldrich Inc., 99.8%, CatalogNo. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,Catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Furthermore, the object of the measurementswas also dissolved in the solvent and adjusted such that theconcentration thereof was 10 mmol/L. Note that in the case where theobject was not completely dissolved, the supernatant fluid was used forthe CV measurements. Further, a platinum electrode (a PTE platinumelectrode, product of BAS Inc.) was used as a working electrode; aplatinum electrode (a VC-3 Pt counter electrode (5 cm), product of BASInc.) was used as an auxiliary electrode; and an Ag/Ag⁺ electrode (anRE5 nonaqueous solvent reference electrode, product of BAS Inc.) wasused as a reference electrode. Note that the CV measurements wereconducted at room temperature (greater than or equal to 20° C. and lessthan or equal to 25° C.).

(Calculation of Potential Energy of Reference Electrode with Respect toVacuum Level)

First, the potential energy (eV) of the reference electrode (an Ag/Ag⁺electrode) used in Example 4 with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation-reduction potential offerrocene in methanol is +0.610 [V vs. SHE] with respect to a standardhydrogen electrode (Reference: Christian R. Goldsmith et al., J. Am.Chem. Soc., Vol. 124, No. 1, pp.83-96, 2002). On the other hand, whenthe oxidation-reduction potential of ferrocene in methanol wascalculated using the reference electrode used in Example 4, the resultwas +0.20 [V vs. Ag/Ag⁺]. Therefore, it was found that the potentialenergy of the reference electrode used in Example 4 was lower than thatof the standard hydrogen electrode by 0.41 [eV].

Here, it is known that the potential energy of the standard hydrogenelectrode with respect to the vacuum level is −4.44 eV (Reference:Toshihiro Ohnishi and Tamami Koyama, High Molecular EL Material,Kyoritsu Shuppan, pp.64-67). Accordingly, the potential energy of thereference electrode used in Example 4 with respect to the vacuum levelcould be determined to be −4.44−0.41=−4.85 [eV].

(Measurement Example 1: YGASF

In Measurement Example 1, the oxidation characteristics of YGASF wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 31 shows the measurement results. Note that themeasurements of the oxidation characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.18 V to +1.80 V and then +1.80 V to −0.18V.

As shown in FIG. 31, it can be seen that an oxidation peak potentialE_(pa) is +0.63 V and a reduction peak potential E_(pc) is +0.55 VTherefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be +0.59 V. This shows thatYGASF is oxidized by an electrical energy of +0.59 [V vs. Ag/Ag⁺], andthis energy corresponds to the HOMO level. Here, as described above, thepotential energy of the reference electrode used in Example 4 withrespect to the vacuum level is −4.85 [eV]. Therefore, the HOMO level ofYGASF can be calculated to be −4.85−(+0.59)=−5.44 [eV].

(Measurement Example 2: YGABP)

In Measurement Example 2, the oxidation characteristics of YGABP wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 32 shows the measurement results. Note that themeasurements of the oxidation characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.20 V to +1.00 V and then +1.00 V to −0.20V.

As shown in FIG. 32, it can be seen that an oxidation peak potentialE_(pa) is +0.66 V and a reduction peak potential E_(pc) is +0.50 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be +0.58 V. This shows thatYGABP can be oxidized by an electrical energy of +0.58 [V vs. Ag/Ag⁺],and this energy corresponds to the HOMO level. Here, as described above,the potential energy of the reference electrode used in Example 4 withrespect to the vacuum level is −4.85 [eV]. Therefore, the HOMO level ofYGABP can be calculated to be −4.85−(+0.58)=−5.43 [eV].

(Measurement Example 3: TCTA)

In Measurement Example 3, the oxidation characteristics of TCTA wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 33 shows the measurement results. Note that themeasurements of the oxidation characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.10 V to +0.80 V and then +0.80 V to −0.10V.

As shown in FIG. 33, it can be seen that an oxidation peak potentialE_(pa) is +0.57 V and a reduction peak potential E_(pc) is +0.49 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be +0.53 V. This shows that TCTAcan be oxidized by an electrical energy of +0.53 [V vs. Ag/Ag⁺], andthis energy corresponds to the HOMO level. Here, as described above, thepotential energy of the reference electrode used in Example 4 withrespect to the vacuum level is −4.85 [eV]. Therefore, the HOMO level ofTCTA can be calculated to be −4.85−(+0.53)=−5.38 [eV].

(Measurement Example 4: NPB)

In Measurement Example 4, the oxidation characteristics of NPB wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 34 shows the measurement results. Note that themeasurements of the oxidation characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.20 V to +0.80 V and then +0.80 V to −0.20V.

As shown in FIG. 34, it can be seen that an oxidation peak potentialE_(pa) is +0.45 V and a reduction peak potential E_(pc) is +0.39 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be +0.42 V. This shows that NPBcan be oxidized by an electrical energy of +0.42 [V vs. Ag/Ag⁺], andthis energy corresponds to the HOMO level. Here, as described above, thepotential energy of the reference electrode used in Example 4 withrespect to the vacuum level is −4.85 [eV]. Therefore, the HOMO level ofNPB can be calculated to be −4.85−(+0.42)=−5.27 [eV].

(Measurement Example 5: DNTPD)

In Measurement Example 5, the oxidation characteristics of DNTPD wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 35 shows the measurement results. Note that themeasurements of the oxidation characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.05 V to +1.20 V and then +1.20 V to −0.05V.

As shown in FIG. 35, it can be seen that an oxidation peak potentialE_(pa) is +0.26 V and a reduction peak potential E_(pc) is +0.15 VTherefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be +0.21 V. This shows thatDNTPD can be oxidized by an electrical energy of +0.21 [V vs. Ag/Ag⁺],and this energy corresponds to the HOMO level. Here, as described above,the potential energy of the reference electrode used in Example 4 withrespect to the vacuum level is −4.85 [eV]. Therefore, the HOMO level ofDNTPD can be calculated to be −4.85−(+0.21)=−5.06 [eV].

Note that by comparison among the HOMO levels of YGASF, YGABP, TCTA, andNPB which were calculated in the above-described manner, it can be foundthat the HOMO level of YGASF is lower than that of NPB by 0.17 [eV], theHOMO level of YGABP is lower than that of NPB by 0.16 [eV], and the HOMOlevel of TCTA is lower than that of NPB by 0.16 [eV]. Further, bycomparison among the HOMO levels of DNTPD and NPB which were calculatedin the above-described manner, it can be found that the HOMO level ofDNTPD is higher than that of NPB by 0.21 [eV].

This implies that by forming the second layer which is the firsthole-transporting layers (second layer 1512 of FIG. 15A or the secondlayer 1612 of FIG. 16A) using YGASF, YGABP, TCTA, or DNTPD and formingthe third layer which is the second hole-transporting layers (thirdlayer 1513 of FIGS. 15A or 15B and the third layer 1613 of FIGS. 16A and16B) using NPB, an energy gap that suppresses transport of holes aregenerated between the second layer the third layer. That is, the secondlayer (second layer 1512 of FIG. 15A or the second layer 1612 of FIG.16A) can suppress the amount of holes injected into the third layer(third layer 1513 of FIGS. 15A and 15B or the third layer 1613 of FIGS.16A and 16B) and reduce the rate of transport of holes.

Thus, it can be said that the element structures of Example 1 to 3 inwhich YGASF, YGABP, TCTA, or DNTPD is used for the second layer which isthe first hole-transporting layer of the light-emitting element of thepresent invention are suitable for the present invention.

Example 5

In this example, the reduction characteristics oftris(8-quinolinolato)aluminum(III) (abbreviation: Alq),N,N′-diphenylquinacridone (abbreviation: DPQd), andN-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA), which were used for the seventh layer which isthe carrier control layer for controlling the transport of electroncarriers (the seventh layer 1617 of FIG. 16A) in the light-emittingelements fabricated in Example 2 (the light-emitting elements 5 to 7)and the light-emitting elements fabricated in Example 3 (thelight-emitting elements 9 to 11), were evaluated by cyclic voltammetry(CV) measurements. Further, from the measurements, the LUMO levels ofAlq, DPQd, and 2PCAPA were obtained. Note that an electrochemicalanalyzer (ALS model 600A or 600C, manufactured by BAS Inc.) was used forthe measurements.

As for a solution used for the CV measurements, dehydrateddimethylformamide (DMF) (product of Sigma-Aldrich Inc., 99.8%, CatalogNo. 22705-6) was used as a solvent, and tetra-n-butylammoniumperchlorate (n-Bu₄NClO₄, product of Tokyo Chemical Industry Co., Ltd.,Catalog No. T0836), which was a supporting electrolyte, was dissolved inthe solvent such that the concentration of tetra-n-butylammoniumperchlorate was 100 mmol/L. Furthermore, the object of the measurementswas also dissolved in the solvent and adjusted such that theconcentration thereof was 10 mmol/L. Note that in the case where theobject was not completely dissolved, the supernatant fluid was used forthe CV measurements. Further, a platinum electrode (a PTE platinumelectrode, product of BAS Inc.) was used as a working electrode; aplatinum electrode (a VC-3 Pt counter electrode (5 cm), product of BASInc.) was used as an auxiliary electrode; and an Ag/Ag⁺ electrode (anRE5 nonaqueous solvent reference electrode, product of BAS Inc.) wasused as a reference electrode. Note that the CV measurements wereconducted at room temperature (greater than or equal to 20° C. and lessthan or equal to 25° C.).

(Calculation of Potential Energy of Reference Electrode with Respect toVacuum Level)

First, the potential energy (eV) of the reference electrode (an Ag/Ag⁺electrode) used in Example 5 with respect to the vacuum level wascalculated. That is, the Fermi level of the Ag/Ag⁺ electrode wascalculated. It is known that the oxidation-reduction potential offerrocene in methanol is +0.610 [V vs. SHE] with respect to a standardhydrogen electrode (Reference: Christian R. Goldsmith et al., J. Am.Chem. Soc., Vol. 124, No. 1, pp.83-96, 2002). On the other hand, whenthe oxidation-reduction potential of ferrocene in methanol wascalculated using the reference electrode used in Example 5, the resultwas +0.20 [V vs. Ag/Ag⁺]. Therefore, it was found that the potentialenergy of the reference electrode used in Example 5 was lower than thatof the standard hydrogen electrode by 0.41 [eV].

Here, it is known that the potential energy of the standard hydrogenelectrode with respect to the vacuum level is −4.44 eV (Reference:Toshihiro Ohnishi and Tamami Koyama, High Molecular EL Material,Kyoritsu Shuppan, pp.64-67). Accordingly, the potential energy of thereference electrode used in Example 5 with respect to the vacuum levelcould be determined to be −4.44−0.41=−4.85 [eV].

(Measurement Example 6: Alq)

In Measurement Example 6, the reduction characteristics of Alq wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 36 shows the measurement results. Note that themeasurements of the reduction characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.69 V to −2.40 V and then −2.40 V to −0.69V.

As shown in FIG. 36, it can be seen that a reduction peak potentialE_(pc) is −2.20 V and an oxidation peak potential E_(pa) is −2.12 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be −2.16 V. This shows that Alqcan be reduced by an electrical energy of −2.16 [V vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, as described above, thepotential energy of the reference electrode used in Example 5 withrespect to the vacuum level is −4.85 [eV]. Therefore, the LUMO level ofAlq can be calculated to be −4.85−(−2.16)=−2.69 [eV].

(Measurement Example 7: DPQd)

In Measurement Example 7, the reduction characteristics of DPQd wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 37 shows the measurement results. Note that themeasurements of the reduction characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.40 V to −2.10 V and then −2.10 V to −0.40V. Further, since DPQd has low solubility and could not be completelydissolved in a solvent even when the solution was adjusted to containDPQd at a concentration of 10 mmol/L, the supernatant fluid wasextracted with the undissolved residue precipitated and used for themeasurements.

As shown in FIG. 37, it can be seen that a reduction peak potentialE_(pc) is −1.69 V and an oxidation peak potential E_(pa) is −1.63 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be −1.66 V. This shows that DPQdcan be reduced by an electrical energy of −1.66[V vs. Ag/Ag⁺], and thisenergy corresponds to the LUMO level. Here, as described above, thepotential energy of the reference electrode used in Example 5 withrespect to the vacuum level is −4.85 [eV]. Therefore, the LUMO level ofDPQd can be calculated to be −4.85−(−1.66)=−3.19 [eV].

(Measurement Example 8: 2PCAPA)

In Measurement Example 8, the reduction characteristics of 2PCAPA wereevaluated by cyclic voltammetry (CV) measurements. The scan rate was setto be 0.1 V/sec. FIG. 38 shows the measurement results. Note that themeasurements of the reduction characteristics were performed by scanningthe potential of the working electrode with respect to the referenceelectrode in the ranges of −0.41 V to −2.50 V and then −2.50 V to −0.41V

As shown in FIG. 38, it can be seen that a reduction peak potentialE_(pc) is −2.21 V and an oxidation peak potential E_(pa) is −2.14 V.Therefore, a half-wave potential (an intermediate potential betweenE_(pc) and E_(pa)) can be calculated to be −2.18 V. This shows that2PCAPA can be reduced by an electrical energy of −2.18 [V vs. Ag/Ag⁺],and this energy corresponds to the LUMO level. Here, as described above,the potential energy of the reference electrode used in Example 5 withrespect to the vacuum level is −4.85 [eV]. Therefore, the LUMO level of2PCAPA can be calculated to be −4.85−(−2.18)=−2.67 [eV].

Note that by comparison between the LUMO levels of Alq and DPQd whichwere calculated in the above-described manner, it can be found that theLUMO level of DPQd is lower than that of Alq by as much as 0.50 [eV].This means that by adding DPQd to Alq, DPQd acts as an electron trap.This is the case described in Embodiment Mode 2 in which the seventhlayer thermodynamically controls transport of carriers (electrons) andis formed of a metal complex which is a material having anelectron-transporting property (Alq) and a quinacridone derivativehaving an electron-trapping property (DPQd).

Further, by comparison between the LUMO levels of Alq and 2PCAPA whichwere calculated in the above-described manner, it can be found that adifference between the LUMO levels of 2PCAPA and Alq is only 0.02 [eV].This means that by adding 2PCAPA to Alq, electrons can be easilytransported to 2PCAPA, and the rate of transport of electrons of thewhole layer is reduced. This is the case described in Embodiment Mode 2in which the seventh layer controls transport of carriers kineticallyand is formed of a metal complex that is a material having anelectron-transporting property (Alq) and an aromatic amine compoundhaving an electron-transporting property (2PCAPA).

Thus, an element structure including a carrier control layer formed of acombination of Alq and DPQd or a combination of Alq and 2PCAPA issuitable for the present invention.

This application is based on Japanese Patent Application serial no.2007-250934 filed with Japan Patent Office on Sep. 27, 2007, the entirecontents of which are hereby incorporated by reference.

1. A light-emitting element comprising: an EL layer between a pair ofelectrodes, wherein the EL layer comprises a first layer having ahole-injecting property, a second layer having a hole-transportingproperty, and a third layer having a hole-transporting property betweenan electrode functioning as an anode and a fourth layer having alight-emitting property, and wherein an absolute value of a highestoccupied molecular orbital level of the second layer is larger thanabsolute values of highest occupied molecular orbital levels of thefirst layer and the third layer.
 2. The light-emitting element accordingto claim 1, wherein the absolute value of the highest occupied molecularorbital level of the second layer is larger than the absolute value ofthe highest occupied molecular orbital levels of the first layer and thethird layer by 0.1 eV or more.
 3. A light-emitting element comprising:an EL layer between a pair of electrodes, wherein the EL layer comprisesa first layer having a hole-injecting property, a second layer having ahole-transporting property, and a third layer having a hole-transportingproperty between an electrode functioning as an anode and a fourth layerhaving a light-emitting property, and wherein an absolute value of ahighest occupied molecular orbital level of the second layer is smallerthan absolute values of highest occupied molecular orbital levels of thefirst layer and the third layer.
 4. The light-emitting element accordingto claim 3, wherein the absolute value of the highest occupied molecularorbital level of the second layer is smaller than the absolute value ofthe highest occupied molecular orbital levels of the first layer and thethird layer by 0.1 eV or more.
 5. A light-emitting element comprising:an EL layer between a pair of electrodes, wherein, the EL layercomprises a first layer having a hole-injecting property, a second layerhaving a hole-transporting property, and a third layer having ahole-transporting property between an electrode functioning as an anodeand a fourth layer having a light-emitting property, and comprises afifth layer controlling transport of electrons between an electrodefunctioning as a cathode and the fourth layer, wherein an absolute valueof a highest occupied molecular orbital level of the second layer islarger than absolute values of highest occupied molecular orbital levelsof the first layer and the third layer, and wherein the fifth layercomprises a first organic compound having an electron-transportingproperty and a second organic compound having a hole-transportingproperty, and a content of the second organic compound is less than 50%of the total in mass ratio.
 6. A light-emitting element comprising: anEL layer between a pair of electrodes, wherein the EL layer comprises afirst layer having a hole-injecting property, a second layer having ahole-transporting property, and a third layer having a hole-transportingproperty between an electrode functioning as an anode and a fourth layerhaving a light-emitting property and comprises a fifth layer controllingtransport of electrons between an electrode functioning as a cathode andthe fourth layer, wherein an absolute value of a highest occupiedmolecular orbital level of the second layer is smaller than absolutevalues of highest occupied molecular orbital levels of the first layerand the third layer, and wherein the fifth layer comprises a firstorganic compound having an electron-transporting property and a secondorganic compound having a hole-transporting property, and a content ofthe second organic compound is less than 50% of the total in mass ratio.7. The light-emitting element according to claim 5, wherein a differencebetween an absolute value of a lowest unoccupied molecular orbital levelof the second organic compound and an absolute value of a lowestunoccupied molecular orbital level of the first organic compound is 0.3eV or less, and wherein a relationship of P₁/P₂>3 is satisfied where adipole moment of the first organic compound is P₁ and a dipole moment ofthe second organic compound is P₂.
 8. The light-emitting elementaccording to claim 6, wherein a difference between an absolute value ofa lowest unoccupied molecular orbital level of the second organiccompound and an absolute value of a lowest unoccupied molecular orbitallevel of the first organic compound is 0.3 eV or less, and wherein arelationship of P₁/P₂>3 is satisfied where a dipole moment of the firstorganic compound is P₁ and a dipole moment of the second organiccompound is P₂.
 9. The light-emitting element according to claim 5,wherein a difference between an absolute value of a lowest unoccupiedmolecular orbital level of the second organic compound and an absolutevalue of a lowest unoccupied molecular orbital level of the firstorganic compound is 0.3 eV or less, wherein the first organic compoundis a metal complex, and wherein the second organic compound is anaromatic amine compound.
 10. The light-emitting element according toclaim 6, wherein a difference between an absolute value of a lowestunoccupied molecular orbital level of the second organic compound and anabsolute value of a lowest unoccupied molecular orbital level of thefirst organic compound is 0.3 eV or less, wherein the first organiccompound is a metal complex, and wherein the second organic compound isan aromatic amine compound.
 11. A light-emitting element comprising: anEL layer between a pair of electrodes, wherein the EL layer comprises afirst layer having a hole-injecting property, a second layer having ahole-transporting property, and a third layer having a hole-transportingproperty between an electrode functioning as an anode and a fourth layerhaving a light-emitting property and comprises a fifth layer controllingtransport of electrons between an electrode functioning as a cathode andthe fourth layer, wherein an absolute value of a highest occupiedmolecular orbital level of the second layer is larger than absolutevalues of highest occupied molecular orbital levels of the first layerand the third layer, and wherein the fifth layer comprises a firstorganic compound having an electron-transporting property and a secondorganic compound having an electron-trapping property, and a content ofthe second organic compound is less than 50% of the total in mass ratio.12. A light-emitting element comprising: an EL layer between a pair ofelectrodes, wherein the EL layer comprises a first layer having ahole-injecting property, a second layer having a hole-transportingproperty and a third layer having a hole-transporting property betweenan electrode functioning as an anode and a fourth layer having alight-emitting property and comprises a fifth layer controllingtransport of electrons between an electrode functioning as a cathode andthe fourth layer, wherein an absolute value of a highest occupiedmolecular orbital level of the second layer is smaller than absolutevalues of highest occupied molecular orbital levels of the first layerand the third layer, and wherein the fifth layer comprises a firstorganic compound having an electron-transporting property and a secondorganic compound having an electron-trapping property, and a content ofthe second organic compound is less than 50% of the total in mass ratio.13. The light-emitting element according to claim 11, wherein anabsolute value of a lowest unoccupied molecular orbital level of thesecond organic compound is larger than absolute values of lowestunoccupied molecular orbital level of the first organic compound by 0.3eV or more.
 14. The light-emitting element according to claim 12,wherein an absolute value of a lowest unoccupied molecular orbital levelof the second organic compound is larger than absolute values of lowestunoccupied molecular orbital level of the first organic compound by 0.3eV or more.
 15. The light-emitting element according to claim 11,wherein the first organic compound is a metal complex, and wherein thesecond organic compound is a coumarin derivative.
 16. The light-emittingelement according to claim 12, wherein the first organic compound is ametal complex, and wherein the second organic compound is a coumarinderivative.
 17. The light-emitting element according to claims 11,wherein the first organic compound is a metal complex, and wherein thesecond organic compound is a quinacridone derivative.
 18. Thelight-emitting element according to claim 12, wherein the first organiccompound is a metal complex, and wherein the second organic compound isa quinacridone derivative.
 19. The light-emitting element according toclaim 1, wherein the fourth layer comprises a substance having anelectron-transporting property.
 20. The light-emitting element accordingto claim 3, wherein the fourth layer comprises a substance having anelectron-transporting property.
 21. The light-emitting element accordingto claim 5, wherein the fourth layer comprises a substance having anelectron-transporting property.
 22. The light-emitting element accordingto claim 6, wherein the fourth layer comprises a substance having anelectron-transporting property.
 23. The light-emitting element accordingto claim 11, wherein the fourth layer comprises a substance having anelectron-transporting property.
 24. The light-emitting element accordingto claim 12, wherein the fourth layer comprises a substance having anelectron-transporting property.
 25. The light-emitting element accordingto claim 5, wherein a thickness of the fifth layer is greater than orequal to 5 nm and less than or equal to 20 nm.
 26. The light-emittingelement according to claim 6, wherein a thickness of the fifth layer isgreater than or equal to 5 nm and less than or equal to 20 nm.
 27. Thelight-emitting element according to claim 11, wherein a thickness of thefifth layer is greater than or equal to 5 nm and less than or equal to20 nm.
 28. The light-emitting element according to claim 12, wherein athickness of the fifth layer is greater than or equal to 5 nm and lessthan or equal to 20 nm.
 29. A light-emitting device comprising thelight-emitting element according to claim
 1. 30. A light-emitting devicecomprising the light-emitting element according to claim
 3. 31. Alight-emitting device comprising the light-emitting element according toclaim
 5. 32. A light-emitting device comprising the light-emittingelement according to claim
 6. 33. A light-emitting device comprising thelight-emitting element according to claim
 11. 34. A light-emittingdevice comprising the light-emitting element according to claim
 12. 35.An electronic appliance comprising the light-emitting device accordingto claim
 29. 36. An electronic appliance comprising the light-emittingdevice according to claim
 30. 37. An electronic appliance comprising thelight-emitting device according to claim
 31. 38. An electronic appliancecomprising the light-emitting device according to claim
 32. 39. Anelectronic appliance comprising the light-emitting device according toclaim
 33. 40. An electronic appliance comprising the light-emittingdevice according to claim 34.